[RPG] Does speaking Giant mean understanding Giant Runes (Storm King’s Thunder)

In Storm King's Thunder, the giant runes are used in magical contexts, as in the various magic items, and as nonmagical insignia—kind of like the no symbol or "Ghostbusters thing." They aren't used to write things down. There are a couple places, such as a location described on page 212, where it's made clear that giants do their sentences-and-paragraphs writing in Dethek (the Dwarvish script), not in giant runes. So it looks like the giant runes aren't a full-fledged writing system, and a fluent speaker/reader/writer of giant wouldn't necessarily know them.

There's more compelling evidence, but it is spoilery:

On page 125, Harshnag "is familiar with the ise rune and can identify it; the other runes are unknown to him." The runes in question are those that correspond to the six giant tribes. Harshnag is a frost giant and on page 120 it is expressed in no uncertain terms that he speaks Giant, but apparently this does not mean he knows all or even more than one of the runes.