Although I would agree with you that there is nothing specifically “Marxist” in US elites’ anti-white policy of mass immigration, black adoration & media censorship, still:

* the only ideological – perhaps unacknowledged- predecessors of current policy were Herbert Marcuse, Erich Fromm and a few other writers from Frankfurt school, albeit marginally (Adorno, Horkheimer, the leaders, did not participate). Here, one must be careful- those “Marxists” have been ueber-European & had disliked racial minorities. Only Marcuse would qualify, with his antipathy towards real working class & exaltation of racial minorities and sexual “liberation”

* serious thinkers who could be connected to contemporary ruling ideology, apart from Gunnar Myrdal- I find hard to locate. Burnham ? Hayek ?
Von Mises ? .. Hardly.

So, I’d say that contemporary multiculturalism is not strictly “Marxist”, but is an evolution of 1970s counterculture which had had some quasi-Marxist ideological element combined with weird “ideologies” like Tantric sex, Wilhelm Reich and deep ecology and whatnot …