The way it’s written now is a comma splice, which is grammatically wrong.

It was a grammatical error. An m dash would have made the sentence clearer. A comma isn’t correct in that case.

Mansplaining refers to a man being condescending and assuming a woman doesn’t understand something. OP made a grammatical error. He pointed it out. That’s really it.

He’s right. Just because a dude says something doesn’t make it mansplaining.