I think it's pretty sexist to ask the question of whether or not it's sexist. It seems like it's honestly a great testament to Jessica's coach to argue for her like this. It shows that he's invested in her life and cares for her both as an athlete and a mother.
None of us men know personally what giving birth is like and the way in which it affects a body physically. Some of us may know what it's like to care for a child in those early months - the lack of sleep, the worrying, etc.
But mostly, given that being the GOAT of track and field is a subjective thing and a relatively small deal in the world at large, it's just a little off-putting that anybody would question this statement as anything other than a coach supporting his athlete. To extend it to the realm of sexism is just jarring, and, like I said, a little sexist itself. It's like, why is that your first reaction? Why can't we simply celebrate Jessica for being both a great athlete and a mother?