Where someone attempts to invalidate anything that’s problematic to their narrative and/or would trigger cognitive dissonance. Conspiracy theorists do this a lot, but so do your garden-variety bigot: when you’re outed as a drug addict, anyone can literally accuse you of anything, because no matter what you say or how stupid the accusation is, “junkies will do or say anything for dope,” “well of course junkies lie,” and “maybe you don’t remember because you were tweaking hard.”
I think you’re mostly right, there’s just a nuance between the “class reductionism” which is wrong because it means disregarding other struggles entirely and “historical materialism reduces History to class struggle” which shouldn’t be read as an insult to Marxist science
Thing is, we got to find an effective way of encompassing History to analyse it or we’re going to get lost in so many details it will lead to stagnation, it’s okay to use reduction in that case, it’s on board with the philosophy of action and change that characterize Marxism
For example, it’s not wrong to say “Marxism doesn’t account for how psychology affects History” but it’s also totally liberal wanking because we won’t achieve anything by psychoanalyzing every goddamn person in the world because taking any action
My point is, reductionism isn’t technically bad, it’s bad when you use it wrong