It was kind of strange for Merc to Battle Piastri so hard and not Battle Max at all considering how friendly Max and Russell are.....

For whatever reason.... Russell decided "Piastri" was a hill he wanted to die on. I also got the impression that his engineer was trying to nudge him to let Piastri through.
The other thing was, had Piastri stayed out when Ant pitted, he would have been in clean air, Russell had pulled ahead a bit. It was NOT obvious that Ant was gonna have a pit stop issue, it was just luck that it wasn't Piastri that had a pit stop delay or that they both had good stops and Piastri just came out behind Ant again.
Anyways, hind sight is 20/20!