Suppose someone with a PhD demanded that everyone always respect them by that title, even when not in their presence, but they used that title to be arrogant and snobby to everyone. It isn't the fact that they have a PhD that's the problem, but I may very well not respect their wishes to be addressed by that title if that's how they treat others.
In the case of my former coworker, it is NOT ok to be disrespectful and short with customers for using the wrong-gendered language than what you'd prefer, when you're not even presenting in any way, and they don't know you and don't know your situation.
"Yes, there are. I have certainly heard the phrase, "you're not old enough to be a Walter.""
The words 'he' and 'she' are much more rigid concepts than stereotypes associated with some names. I've never met a white Tyrone, but being black isn't intrinsic to the name Tyrone. On the other hand, appearing to be a woman is an intrinsic implication of the pronoun 'she'.
"Yet, online we just accept everyone's self chosen username."
Because online usernames and the name you use in real life are 2 very different things. The internet gives the cloak of anonymity, and it's ubiquitously expected that people will go by fake usernames that mask their identity. People who don't know you online who you offend can't cause you harm in the way someone in your proximity IRL can cause you harm. Going by an extremely absurd or overtly offensive name IRL is generally not accepted. Respecting someone's wishes to be called Ni**erWhipper, or Adolf Hitler II, or PreschoolGirlF***er could put me in physical danger.
"We might abbreviate it out of sheer laziness but if someone named themselves StraightWhiteMaleSlayer that would be the only way to refer to them."
Yeah, not going to respect someone's wishes to be called that, or any abbreviation of it.