As you'll probably recall it's not just the anti-trust exemption, but MLB actively getting Congress to pass an additional law (with help from the MILB office and owners, which it then abandoned).
It's tricky to enforce in any case, since most things are still rigged against employees. Like, a lot of salaried office workers paid based on an eight-hour day probably work 10-12. Bar and restaurant workers in tipped minimum wage states still work an hour before and after their shift not earning tips. And baseball players basically aren't paid for their time in the gym and cages and meeting rooms.
Do these players even get paid the rest of the summer if they're not in an actual league? Housing or per diem?