I think what matters is what they agreed to, though, right?
For example, if the Celtics said "our guys say IT will start season on time, you've seen the med records. We will offer the Nets pick if you take Thomas as-is and assume the risk he doesn't make our guys projection" and Cle agreed then it isn't about Cleveland's medical staff at all, imo---they waived any additional concerns about his physical, effectively, at least as to the hip.
I doubt it's quite that clear what they agreed to, of course. What I suspect is they Celtics disclosed records, their doctor's best assessment, and some form of limitation on his health. Cleveland--whether legitimately based on their medical evaluation, purely tacticaly, or something in between--is now disagreeing with the premise of that limitation. Perhaps Cle said 'we agree so long as our docs concur with your docs' in which case theyy are entirely justified in disagreeing. Perhaps they said "absent new info different from his records we agree" and the debate now is about whether anything in exam is new, which feels like a resolvable situation (one way or the other).
I don't think it's obvious which side has the better argument there, and we just need to understand the commitments/understandings more.