It's fine to use he/him for a male pseudonym.

However, in an article in which you speculate about the real identity behind a pseudonym, suggesting any possible gender or plurality, you should use /they/, this being the whole point of what your doing.

Can you link a style guide that captures this norm? I usually consider pseudonyms to function like a pointer to the real person when the real person is in scope. If my HN handle was JaneAustism and I was being referenced in a discussion about things pertaining to my meatspace identity and I was a male, I would expect to be described with masculine pronouns. If the discussion was not related to my meatspace identity, I would, in a pseudonymous context, not expect to be referred to as she/her just because my handle contains a female name. My gender is likely irrelevant (or at least unknown) in that context and I would expect they/them.

You seem really upset about this. Why does a pronoun choice bother you so strongly?