It depends on the fact dependent nature of the worker relationship. I think you are right, in most cases, they will be 1099 contractors responsible for all their own income taxes and self-employment taxes.
There could be a lot of planning opportunities for many of these athletes, such as s corporations, if they are aware to consult competent tax professionals.
I don't think there's ever been any doubt that contributing to an NIL collective is absolutely not a charitable donation. And for business owners, it's much better to work out your own deal with an athlete if you want it to be an advertising or marketing deduction.