Mark has been pointing out the flaws in Alex's playing (at least how he perceives them) for quite some time now. I haven't seen them live, so some of the points he made might be more audible then. But I also checked different recordings here and there and can't agree on that basis. Also, it has become a running gag sort of thing because Mark is usually a super chill dude about everything LP related, but his criticism of Alex has turned a bit comedical at this point.
I agree with you, though, that Brad (aside from his way of playing and thus "sounding" on stage) was notorious for being sloppy since forever. Overall, Alex seems much more on point from the small sample size I checked.