This is a textbook case of a troll derailing an entire conversation and making it all about themselves. What sort of person concern-trolls a memorial post and argues with mourners? Before the internet, a person like that would have to crash a funeral to reach that level of abusive behaviour.
Now. I know next to nothing about that subculture and even less about that specific community. But I doubt very much that this individual is motivated by genuine concern over “cultural appropriation”, especially when there is little or no evidence to support their accusation. Far more likely to be a case of clinical-level personality disorder, and has appropriated the rhetoric of “Tumblr social justice” because that’s a known weak spot in some communities’ immune systems. In other words, it’s pure trolling in the original sense of the word.
There are plenty of reasons to criticise the memes of Tumblr-style social justice, and one of those is the way it creates such a rich environment for trolls. It can be hard to tell the different between trolls and zealots with no clue about boundaries, and that allows trolls to thrive.
Trolls are parasites. They suck up attention and energy that could have been used in sharing information or building community, while contributing nothing in return. Their “free speech” comes with too high a price tag for anyone else.