Multiculturalism, in the speech and writings of those fervently espousing it, itself often excludes white people in the US, and can be rejected. While assimilation in the sense of fully adopting another's customs might be extreme, people can, and whole groups of people have, assimilated successfully in the US without giving up their own culture.
Multiculturalism is a good thing, but assimilation comes from different cultures blending into one. Forced assimilation is bad, but the assimilation that occurs freely is not a false standard. It is the result of different cultures learning from and respecting each other and
blending to create a new type of culture.