Yes, it is that simple. You're using a word that you don't know the meaning of and in doing so not recognizing the direct and obvious contradiction in your use of the word in this context.
Thanks for asking for the explanation, I'll take this opportunity to do so. I am not a post-modernist myself, and this is not a defense of post-modernism, but since I actually know what the fuck I'm talking about I'll use this opportunity to provide some illumination instead of letting people butcher the use of language.
Post-modernism is not a political ideology, and it is not just one 'thing'. It is an umbrella term, under which many views and ideologies sit beneath. It is a lens through which to see the world, like modernism.
That being said, there are some connecting ideas and themes that can be attributed to all realms of post modern thinking. The main themes and ideas are this - there is no "objective" reality that can actually be understood. Virtually everything is an issue of perspective, and there are no grand narratives in history. Everything boiling down to 'oppressor vs oppressed' is a grand narrative, and is in direct contradiction to post-modern theory.
I hope that helped. If you want to use the term "Marxist", fine. Marxism is still much more complicated for your reactionary smooth brains to ever fully grasp - but if you want to accuse the left of using a "Marxist, oppressor vs oppressed" ideology, that would be much more accurate than calling them post-modernists. You might as well say "the left are using an Existentialist oppressor vs oppressed ideology!!!" Both are equally wrong and equally disconnected from the actual meaning of those words and ideologies.