Technically it implies that all these other races are diverged near humans, humans being relatively unchanged remain close enough to produce viable offspring, but with different non human races being diverged from each other to the point of non viability.
So basically the racial map for a D&D setting would have humans at the center, with half children in each of the spokes of a wheel, and every non human race being nodes located in the environment where they developed in extremity, and then from there you can build the environment under the premise of the conditions that developed elves or dwarves or orcs from the human starting point.
This would also have to include a backstory spanning tens of thousands of years.
Alternative: humans were specifically engineered to be able to half-breed with anything - even elemental beings - so that they’d be able to take over the world.
Huh… never thought of that. Though I think a key difference is that it’s one race diluting many races, rather than… well, in great replacement theory, it’s not even whites being diluted by other races, it’s them being replaced by way of high immigrations and low birth rates. So if it was like a large group of humans migrating into an elvish city, then yes, but this is more like the elvish country gaining a population of half elves and eventually humans around the edges.
From what I can tell in the wiki, great replacement people aren’t so much threatened by half-minorities as they are by flocks of minorities moving in until whites are the minority. It’s the culture shock, and you don’t get as much of a culture shock from someone who was raised on the edges of your culture.
Not to say you have to include human hegemony in your campaign, of course. Your campaign, your rules, and you know what your players are comfortable with more than I do.
