This is correct as marxism and post-modernism overlap but are ultimately opposed.
Post-modernism and marxism overlap in treating ideas as man-made and can criticise them as propaganda tools for power structure. Hence the overlap in critical theory in treating culture as a form of production of ideas. Who controls the production of ideas, therefore controls a great deal of social consciousness.
However, (Orthodox) Marxism is materialist and believes in an objective world as a source for objectively valid truth and knowledge. The marxist conception of Ideology is like looking in a distorted mirror- yes, it reflects an objective reality but not with 100% accuracy and considerable distortion depending on which position from the social hierarchy you are looking at the mirror from. Some aspects of reality are exaggerated whilst others are under-emphasised depending on each persons experience within the same system. It doesn’t treat it exclusively as a product of the mind or pure illusion as a “social construct”.
There are currently major feuds going on amongst the far left on this issue and how far you can take the identity politics of gender, race, sexuality, etc before it becomes anti-marxist. Its all related to where the boundary between social criticism of post-modernism ends and marxist social criticism begins.
This discussion may be a bit over my head, although I do recall some discussions in my linguistics classes about words and their relation to objective reality.
For example, there's a stick coming out of the ground in my yard - and for English-speakers, this thing is known as a "tree." But the word "tree" itself is not the reality. The word is a marker, a symbol to represent the object - but not the actual object. So, while there may be a truly objective reality out there, humans have insufficient means to perceive and describe it, since words are imperfect reflections of human thought.
A scientist might study the tree, categorize it, and label it with its official scientific term (usually in Latin, to make it sound more high-falutin'). That might give a more accurate description for the sake of clarity, but it still may not be a 100% perfect description of objective reality. But that may be the best we can do.
Could relativism simply be a way of saying that no human being is perfect?