Gearbox are serious. And as to why, deliberately misgendering people can, and may, cause offense to people using the forum.
From the forum rules.
no racist, sexist, homophobic, or transphobic slurs, and other such behavior detrimental to conversation.
For brevity they didn’t list every single thing that the rules covers, but it does extend to them all. I know in the UK at least there’s 9 “protected characteristics” that people can take action if they’re discriminated against for. They include Sexuality, Gender and Race.
Ultimately, referring to FL4K as a “he/him/his” knowingly is misgendering. As was said, the odd slip-up will happen, that is understandable.
A fictional character wrote (either in part or in whole) by someone who, unless I’m mistaken, is non-binary themselves and as such has the right, as their creator, to dictate.