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.