As an Asexual, can confirm, they're just making a joke with a text based stutter, not referring to Asexual or Agender, such as in "LGBTQIA+" that generally refers to multiple things under the "A".

In the latter case, it's left open ended for inclusivity, rather than arbitrarily excluding other things by specifying.

As for how the original comment could possibly be more inclusive, it's harder to say exactly, but I can think of something like "Ladies, Gentlemen, and everyone else", but I'm not exactly great at fancy words... or words in general... but I try my best.

All good, you did nothing wrong haha. I don't think the other person was even genuinely criticizing your post, I think it was meant to be a lighthearted joke.

Besides, asexuality would be a lack of sexual attraction - I guess "agender" would be the only "a-something" that's relevant to a male/female dichotomy.

Asexual usually gets shortened to "ace" I believe.