How can you be a marxist and a relativist at the same time?

It seems pretty clear that the same people who buy into cultural and moral relativism, who see no moral difference between Saudi Arabia and the United States, between Iran and Israel, who claim that you can’t judge any culture as inferior or superior, who direct post-modern deconstruction at nearly or literally every social norm or value, also nearly or exactly overlap 100% on a ven diagram of people who believe in the modern redefinition of the marxist social constructs of oppressor vs. oppressed, which is as clearly a black and white notion of categorical good and evil as you can get.

How do they manage to hold both in their minds at the same time? Is it simply cognitive dissonance? Or do they experience no cognitive dissonance because there is no cognitive awareness of what should be dissonant, to believe both A and -A at the same time?

It just occurred to me reading Icarus Fallen (as it has before, but less clearly) that these  two groups are the same people, and the way Chantel Delsol describes it really shines a light on this contradiction:

“We are here in the realm of a ‘black market’ moral code, one that operates without clearly declaring itself, hiding behind the mask of the philosophy of values. It proposes an objective good while at the same time rejecting the objectivity of the good. It ostracizes those who contest it even while it proclaims subjectivism.”

How is this philosophically possible? Or is it not, and it is simply humanly possible because we are so easily self-deluded?