Is the credit assignment problem a well-posed one?
If you can do it well, it is useful.  If you can accurately identify which move was to blame for bad outcomes -- or which one is to credit for good outcomes -- that is useful in helping you learn to make better moves.  I'll let you decide whether that's enough for it to make sense to speak of it as a problem to be solved. I'm not very knowledgeable about philosophy, so I don't know whether that qualifies.