a man's first serve is massively more threatening than a woman's. Not having a second serve shuts that down

I had to look this up to understand it. For anyone else confused: if you fault on your first serve, you get a second. If you fault again, you lose the point. So people go all out on the first one. If they get a fault, they do a more careful one because of the risk of losing the point. For this match they changed it to fault once and lose the point. This change to the rules made them both rein in their ONLY serves but obviously he's going to be sacrificing way more power between all-out and precision.