(it is more like a thought experiment of what if other hominids survived and co-existed with humans, with their own cultures and languages)
We are saying moreorless the same thing here.
Tolkien posits a thought experiment: "what if other hominids coexisted with their own culture and languages?"
... But the more these "hominids" resemble the species
sapiens, the more Tolkien relies on the pseudoscience of his era to characterize these "hominids" as if human "races".
I see demihumans much more in the realm of being different species.
I also view Elf etcetera as different species of life. In some ways, they are radically Nonhuman.
For example, the Fey Elf is a spirit of the immaterial Fey plane. There is no flesh-and-blood. There is no DNA. There is no biology.
But the Elf can still a 5e "Humanoid".
They a human-like because they are intelligent hominids.
Nordic traditions color my view here.
In the Norse animistic traditions, mountains, lakes, rivers, sky, sun, sunlight, and daylight, are all conscious living persons.
Elves are the army of sunlight. Literally.
So when this sunlight takes the form of a human of flesh-and-blood, the sunlight is using the same magic that human shamans use to take the form of a wolf or falcon.
While in human form, the sunlight actually is a human, and can reproduce offspring genetically with a human, albeit typically exhibits some telltale evidence of the true form being the sunlight.
Likewise, when a human takes the form of an other animal, the animal typically evidences the true human form, such as human eyes in an otherwise normal animal.
but clearly all humans are a single race in D&D, as you point out, which means that encompasses all the human ethnicities.
Fortunately! This is the saving grace of D&D!
I think that is one thing that shows pretty clearly these are creatures of a much more substantial difference from humans (than being similar to the differences among humans).
Plus, for a fantasy gaming, figuring out ways to carry out thought experiments about what Nonhuman lifeforms is part of the fun.