Not sure they believe it, but they try to convince others of it. This way, they can get the lumpenproletariat to fight against and weaken the dominan(i.e., successful) culture, and when the time is right, these multiculturalists can step into power. Look at the poverty and race pimps in our own country.
I think you give them both too much and not enough credit.
Most people with this POV will never enjoy personal political power, and they know it.
Yet they loathe and despise America and Americans. (Of course, the very fact they feel this loathing excludes them and all who feel like them from the guilt of being American.)
The fact that their core belief is anti-Americanism is the reason they can spin on a dime from being socialist to being multiculturalist or environmentalist.
They don’t hate American because it isn’t multicultural or environmental or socialist. Those are all mainly after the fact rationalizations to explain to themselves the hatred they feel. If one rationalization falls on its face they don’t reexamine the hatred, they just pick up another rationalization.
The whole thing is very odd to me, as America has objectively provided greater freedom and prosperity to more people than any society in history. Seems like the logical thing to do would be to celebrate that fact, not despise the society that created it.