Marxists disagree with "anarcho-antirealists" because Marxists are materialists, not idealists. Grail is stating that reality itself is a reflection of ideas, and that, therefore, escapism is praxis. Marxists take the opposite approach, and seek real liberation by real means, as dialectical materialists. Propaganda and cultural hegemony have little to do with why Marxists oppose idealists, it's because idealism itself is bourgeois and prevents effective praxis (see escapism as being seen as praxis by idealists, rather than organizing and struggle).

Also not sure what you mean by Marxist-Leninists depending on "propaganda and manipulation," we certainly believe in dispelling bourgeois mythos and in espousing correct, scientific lines on revolutionary struggle. However, the idea that this is in service of some nefarious, manipulative ends is ill-founded. Communists have always fought for the working classes, and against bourgeois cultural hegemony in favor of proletarian culture and science.

It seems more like you're letting your distaste for Marxism cloud your interpretation of Their comment, when They are quite literally telling you They are rejecting material reality in favor of fantasies. You're interpeting the message as getting in control of perceptions to dispel propaganda's effect when the reason the concept is controversial is because it's a fundamentally solipsist ideology.

The place where it gets controversial is that we say gender, race, species, and divinity are also social constructs. Neither inherently true nor false, but rather a choice we make as society, and should therefore treat with responsibility and kindness. We piss off the transphobes by saying trans people exist, the kinphobes by saying otherkin exist, the human supremacists by saying animals deserve rights, the atheists by saying the gods exist, the monotheists by saying ALL the gods exist, and the white realist liberals by saying indigenous myths are literally true, and the rainbow serpent truly did make Australia, North America truly is on the back of a giant turtle, and the big bang truly did create the universe, although Elohim created Jewish people.

We believe everyone about their own cultural truths, and that pisses off all the exclusionary realists.