I've always assumed that the goliath race are half-giants or pygmy giants or something like that. Something related to giants.
In the descriptions of the goliath race in 5e (in Volo's Guide to Monsters and Elemental Evil Player's Companion), it doesn't actually mention the race's origins, nor does it mention their relation to giants at all. It mentions that they know the Giant language and they are described physically as per the following:
Their bodies look as if they are carved from mountain stone and give them great physical power.
The rest of the description largely concerns their society and how they tend to think and act, etc, but does not elaborate any further on what they are or how they came to be. This may be because the descriptions are attempting to be setting-agnostic, so they don't want to tie the race to their origins in the Forgotten Realms, but that leaves me with not much to go on regarding what the goliath race actually is.
I have heard that they are specifically related to stone giants, but since 5e doesn't mention this, I'm unsure if this is derived from older editions of D&D or what? The Forgotten Realms wiki page, at least, mentions this under the "History" section:
It's unclear how old the goliath race was or where they originally came from, but it was commonly believed that they were somehow related to stone giants or earth genasi.[8] Another origin story claimed that the first goliaths were humans who sought answers from their gods. These individuals climbed the Columns of the Sky mountain range to seek parley with their gods. It was said that the journey to the peaks turned the seekers into the first goliaths.[28]
Basically, what actually are goliaths? What is their race's origin? Do they have any relation to giants in terms of "genetics†"? I'm interested in in-universe Forgotten Realms lore from any edition of D&D, not out-of-universe explanations for what might have inspired the designers to come up with the race.
† Note that "genetics" isn't exactly the right term to use for magical fantasy universe races, but I use the term to make it clear that I'm not interested in the "relationship" between goliaths and giants from a social point of view; I don't care what they think of each other, I'm only interested to know if goliaths are somehow derived from giants or not, in-universe.
Best Answer
Races of Stone (3.5e, 2004) introduced the goliaths to D&D, and makes no mention of the Realms. It presents information about goliath culture, deities, and settlements, including the city of Thella-Lu, with no information about fitting these details into existing worlds. This was normal for 3rd-edition setting-agnostic books like Races of Stone. On some level, the default setting in these books was kind of “Greyhawk with the serial numbers filed off,” since most of the deities, and some other details, come from Greyhawk, so maybe Thella-Lu is in Greyhawk. Maybe.
Champions of Valor (3.5e, 2007) is a Forgotten Realms supplement that includes a “Races from Other Supplements” section, including goliaths, on page 17. There isn’t a lot though, and it’s very much presented as ideas for a DM to include them, rather than canon about the Realms.
(Champions of Valor pg. 17)
Literally only that last line is a statement of fact rather than speculation into a possibility, which maybe means that goliaths and earth genasi are “canonically” cousins, though I find it hard to credit it that strongly seeing as it was following up so many “possibilities,” including the original introduction of the idea that goliaths and earth genasi are related.
The mention of Uthgar is relevant, because it gives strong indication that the goliath pantheon is not found in the Realms, which makes sense given how crowded the Realms already was with deities. Then again, lots of races of racial pantheons in Faerûn, and it seems a little unfair that goliaths don’t get to have one.
The city of Thella-Lu also goes unmentioned, though it could presumably be found in, say, that volcanically-warmed hidden valley or whatever.
A couple of Forgotten Realms novels, Depths of Madness by Eric Scott de Bie (2007) and Sandstorm by Christopher Rowe (2011),1 feature goliaths. I haven’t read them and don’t own them, so someone else will have to follow up on those. The footnotes on the Forgotten Realms Wiki article that you link indicates that non-canon statements by de Bie and Thomas M. Costa indicate that Thella-Lu is not in the Realms, and the goliath language is written in the Dethek script.
Between those two novels was the release of 4e, and goliaths appeared in Player’s Handbook 2 (2009). That I do have, and it doesn’t say anything about the Forgotten Realms, for goliaths or anyone else. The default setting for 4e was Points of Light, unrelated to any other D&D setting, and the Realms only got covered by few books in that edition, primarily Forgotten Realms Campaign Guide and Forgotten Realms Player’s Guide (both 2008)—before Player’s Handbook 2 and 4e’s goliaths even existed. Astoundingly, despite that, Forgotten Realms Campaign Guide actually does mention goliaths:
(Forgotten Realms Campaign Guide pg. 182)
(Forgotten Realms Campaign Guide pg. 183)
But those are literally the only mentions in the book.
4e also had a Neverwinter Campaign Setting (2011), which includes Eric Scott de Bie on the cover and includes a goliath—a goliath. “The warden of the sanatorium is Torlgar, a hulking goliath (Monster Manual 2),” and that’s all we get. Torlgar is the warden, is a “hulking goliath,” and is charmed by a one Rohini, who is turning him into a “foulspawned berserker (Monster Manual).” He doesn’t get stats, much less a detailed history of his people. That Monster Manual 2 entry is rather bare, in case you were wondering—literally three sentences of description, none of it remotely about their origins much less the Realms. Torlgar presumably should use the stats for the goliath sunspeaker or the goliath dawncaller found there, but Neverwinter Campaign Setting doesn’t seem to specify which.
Beyond those, there were the adventures for the Living Forgotten Realms organized play program—of those I have, only one mentions goliaths, and that one has used some goliath monster stats “reflavored as hobgoblins.” Dragon and Dungeon magazines were both fairly substantial sources of supplementary information for D&D 4e, and I don’t have a collection of the 4e issues of those magazines, so it’s possible that there was more information about goliaths in the Realms somewhere in there. I kind of doubt it, though.
And then we have 5e, which is kind-of-but-we-swear-it-isn’t set by default in the Forgotten Realms. Sword Coast Adventurer’s Guide is Realms-specific, but doesn’t touch on goliaths. As you note, the books that do, don’t discuss their placement in the Realms. So that’s all we’ve got for now.