Why would the NFLPA go to battle for a non member?
It wouldn't be about fighting for a nonmember, it would be about enforcing the CBA. Sorsby can't sue or seek arbitration based on a violation of the CBA as he isn't a party to the agreement, only the NFLPA has standing.
But there is nothing in the CBA that gives the NFLPA a say in a supplemental draft. The CBA gives the league the sole authority to decide whether or not to hold the supplemental draft and there is nothing that requires one be held just because someone says they want to be in it.
I suspect Kessler will explore other avenues here -- some quotes out there from him suggest that he feels the league was less than forthright about the process. I have a hard time seeing what legal theory could reasonably result in a win, though. And at this point, any win is going to be monetary, which means money out of his hoped-for employers' pockets.