I think you're giving doctors and their advanced technology tooo much credit for being able to clearly define his, (or many) condition(s).
Medical advances are amazing but they can't and don't see and diagnose everything and if they do think they know it's not 100% in most cases.
They could have honestly cleared him to play and just not been able to see a problem. Even with the advanced state of medical technology it's still somewhat of a <crap shoot.