If by white genocide they mean white people will cease to be a thing because of mixed breeding, I do think that will eventually happen. I don't see why it should be seen as a bad thing.
It makes the human species more biologically vulnerable to extinction. A biologically diverse species can better survive under duress from the environment and/or attack from pathogen.
But the human species will soon be extinct for other reasons anyway. So it probably does not matter in the long run.
Interbreeding doesn't make the species less biologically diverse. It makes the diversity less distinguishable.
In fact, it makes the whole system less biologically vulnerable to extinction.
Imagine there are two populations, Red and Blue. Red is immune to Bluepox, and Blue is immune to Redpox. Let's imagine for a moment that each disease targets a different biological vector.
Now, we can see that if there is an epidemic event of Bluepox among the red population AND an epidemic event of Redpox among the blue, we have an extinction happen. Even if the epidemics happen at different times.
If you interbreed red and blue, some of the offspring will have the blue immunity gene, and some will have the red immunity gene. Some children will have neither. Some children will have both.
In this scenario, if there is an epidemic event of bluepox, all of the pureblood Blues will die, but half of those children of mixed parentage will survive: there's a 50/50 chance that Bluepox immunity will be inherited from the Red parent (assuming it's a dominant trait, but whatever This is a thought experiment).
Now, later, when Redpox ravages the population, all the pureblood Reds die. And half of the mixed children do too; the ones not lucky enough to acquire the Blue parent's immunity to Redpox.
This means that of all the population, only children of mixed heritage survive. Long Live the purples.
Many different resistances work this way, both disease and environmental resistances. Only by SHARING those resistances and getting what synergy may exist between them can you get the most rugged population.