You ask a good question. The key element is that the "specific production company" is/was a league member and a party to its collective bargaining agreement. There are no other agreements; everything agreed to between a producer and an actor is subordinate to (or maybe part and parcel of) the CBA. Another key point is that nothing is being done retroactively. Any enforcement of the new agreed up provision would be in the future.
I also think the NDA was probably illegal anyway, but I don't really want to go there. Because we don't need to.