When it comes to a political idea like Marxism labeling it as dangerous doesn't mean that one doesn't consider it stupid or wrong. Stupid is also not a good word to describe a complex argument that you consider to be wrong.

When it comes to Sokal, Sokal had no skills as being well educated in postmodernism. In his idea the postmodernists should have noticed that what he says doesn't make any sense. Roko on the other hand is smart and does have an understanding of the local memespace. Roko isn't stupid or ignorant of the ideas he was discussing.

Thinking about blackmail and how to structure a decision theory to avoid being subject to blackmail is also not a worthless endeavor.