The point of immigration is not just to pump population it's also to make the native population politically weaker
Common ethnicity is a way for large numbers of ordinary people to coordinate with each other. A mixed population can't coordinate and don't see each other as allies

Homogeneity is good for ordinary people and hence bad for elites because it means elites can't get their way as much, and it is also a way for elites to weaken other elites who have ties to the native population.

It should not come as much of a surprise then that many important elites favor immigration, "cultural melting pots", etc.
This effect works even better when you have significant human biodiversity because it makes it much easier to play factions off against each other.

"Enoch was right", but I don't think he quite understood the game theory of why it was happening.