Those ethnicities are specific to Faerûn; other settings have their own ethnicities.
As you correctly surmise, the Player's Handbook clearly defines the nine major ethnic groups as specific to Faerûn, that is to say the Forgotten Realms, while the Sword Coast Adventurer's Guide contains further information specific to that same setting, given that the Sword Coast is a place specific to Faerûn. The Imaskari, for example, are descended from the people of the empire of Imaskar.
While officially no one setting is the "core" setting for D&D 5th edition, in practice, the Forgotten Realms is given focus in the Player's Handbook, considering that its human ethnic groups are listed there.
However, other D&D settings have their own ethnicities.
For example, Greyhawk. The prevailing ethnic groups are the pale-skinned Suel, the golden-skinned Baklunish (who destroyed each other's empires in an ancient magical cataclysm), the Flan (natives to the Flanaess, the primary continent of the World of Greyhawk setting), the Oeridians (founders of the Great Kingdom), the dark-skinned Olman, and the river-dwelling Rhenee. None of the Forgotten Realms ethnicities appear in Greyhawk, although someone can canonically travel between those two worlds.
In Eberron, humans have varying skin, eye and hair colour, but the setting glosses over the exact details, and does not name specific ethnic subgroups. According to a comment by Eberron creator Keith Baker:
This is a case where canon Eberron simply doesn’t make an effort to accurately model demographics in our world. The premise is that your human character can look like what you want it to look like, and we aren’t concretely mapping skin color to region; essentially we are looking at HUMANITY as a “race” and cosmetic variation within humanity as a player choice. It’s not realistic, and within your campaign you can certainly decide to do otherwise, but it’s not something that will be defined in canon.
Dragonlance has its own ethnic groups detailed here including the Abanasinians, Arktos, Cobar, Ergothians, Horselords, Ice People, Istarians, Kazar, Kharolish, Lemishite, Nerakan, Nomadic Humans, Nomads of Khur, Nordmen, Schallsea Folk, Solamnics, Tarmak, Tarsian, Thenolite, Uigan, and Wemitowuk.
Best Answer
The overall demographics of modern Forgotten Realms, ie. say the last couple centuries, seem not to have changed much over the various editions of the game. Since around -3000DR, when the human archmage Iolaum raised the first enclave of what would become the Netherese Empire, humans have been increasing in influence, while the dwarven and elven empires have declined ("Age of Humanity").
For individual cities, detailed statistics are available in Forgotten Realms Adventures (2e) and Forgotten Realms Campaign Setting (3e FRCS). Certain locations, like Evermeet or Luiren, are exclusively or mostly non-human (elves and halflings respectively); others are mentioned to have significant non-human populations like Lantan (gnomes). However most of the settlements on the surface of the continent of Faerun are predominantly human. You can get some of these data (even exact percentages) on the FR wiki.
If you are interested not in particular locations, but in the overall percentage, we can say that humans are the dominant species. In order to get data about the whole planet altogether, it would be meaningful to refer to a sourcebook that considers the whole planet as a unit, as it is observed literally from space: In the 2e game accessory Realmspace, the population analysis is summarized (page 18) as follows: "Human and humanoid races most prevalent".
Finally on the RPGnet forums, a poster with the nickname Handigar claims to have added up all the data from FRCS. Quoting from that post: