this 'death of the author' thing is classic reddit-ism championing less engagement and less intellectual rigor in our media and art discourses,

Depends on how its used. If anything I see things like "your autism headcanon is wrong because the author didn't intend that" as a reddit-ism.

Sometimes its used that way, sometimes its not. Most people citing it have no idea what the entirety of Barthe's theory was anyway. They can mean anything from "I dont think I should have to engage with the reality of the LIVING author's intent, the blatant bigotry in the work, and the actual hateful things they are actively doing in the world" to "I dont care if its intended, [character] reads [neurodiverse or queer] to me".