There are 2 key points about this rule I am thinking: (My credentials as a legal advisor are well established on this forum in that I have zero credentials as a legal advisor so this is all internet-lawyering )
1 - The spirit of the first filed rule is to stop someone from, like, waiting a year and then filing in another jurisdiction either as a delaying tactic or because that person is losing the first case. Obviously that's not what is happening here
2 - Was the NFL's filing actually done first? As we all learned, they re-filed the case today. So both cases were filed on the same day. I can't find a timestamp on the NFLPA's filing (and I haven't seen the NFL's filing). Does the first-filed rule actually go so far as to break down into the actual time of day the motions were filed?