I once found usenet messages from my granny in web archive - probably long lost now (she was already dead when I found them, and they were at least a decade old then - she was the tech-savviest granny I ever knew). She was engaged in a fierce debate about religion, so telling people they're wrong on the internet must run in the family.

Yeah that was me, and now I made you suffer with me! Mahahahaaa!

But to be honest I found the whole thing quite entertaining. It's a puzzle working out what the hell he believes, because he doesn't actually have the level of awareness and understanding to state his position clearly. So every conversation he has on this it's five levels of back-and-forth before he reveals that actually he thinks that, in working out an expression like a(b+c) the very first thing you must do is apply the distributive property to turn it into (ab+ac), because he has got it in his head that the term "multiplication" only applies to multiplication with an explicit symbol like a×b, and if there is no symbol he thinks it's completely different. Hence when order-of-operations rules say "multiplication" after "brackets" he thinks that does not apply to the aforementioned case. And then his lack of awareness comes in, because it's not just that he doesn't realise he's wrong; he doesn't even realise that nobody else is doing it that way.

I'm still not completely convinced he's not a troll, but his dedication across platforms* to this one topic, and his lack of versatility as regards trolling on other topics, pushes it onto the side of sincerity.

*I googled his username and have seen him push this line on GitHub, math.stackexchange, and Mastodon - where he also has pet topics about how 0.999... ≠ 1 and how Cantor's theorem on the uncountability of the reals is wrong because, get this: infinite sets don't exist - both of these being more entertaining but worse in terms of disinformation