His brand is literally "Mr Doob"

The consensus in English-writing guides like the APA or Elements of Style is about staying true to a person's self-identified pronoun while preferring specificity over ambiguity. In a case where you don't know - "they" is perfectly acceptable. In a case where "they" is an explicit self-identity - it's the only correct option. In this case, where he literally calls himself "Mr", it's stylistically incorrect to choose a more ambiguous pronoun where "he" is the correct one for someone calling themselves "mr"

Hm. Interesting argument, but I kinda disagree with the premise that "if we have anything exposing gender metadata, the rest of the language around it in a sentence must be pulled into referencing that to be more correct".

It's a coherent thing to argue for, I'll give you that, but it's also.. kinda arbitrary? And one can make an equally convincing case for statelessness in the sense that just because one word carries metadata, doesn't mean that the other words around it must react to it.