https://quayhealth.com.au/kawhi-leonard-quadriceps-tendinopathy/
I'm not taking a singular medical opinion as truth necessarily, but I think it's possibly a misdiagnosis and his injury could be pertaining to his patella rather than his quad. That and from what I understand, rehab from quad tendinopathy has to address not just the injured area, but the lower leg which would be compensating for tendinopathy. These are possibly things missed. Or,well, it could be a bit more complicated than I'm making it out to be.
Not an athlete, but interesting case study:
http://www.completesportscare.com.au/2014/10/tendinopathy-case-study-unresponsive-quads-tendinopathy/
Medical discussion of Kawhi's injury:
https://www.cbssports.com/nba/news/medical-expert-discusses-kawhi-leonards-mysterious-injury-and-whats-next-for-him/
Kawhi is clearly frustrated with his progress, which is understandable, especially considering how Parker came back with a more serious injury than him. Thing is, a torn tendon addressed w/ surgery is more cut-and-dry. At the same time, the whole thing is still odd.
I'm not putting the full blame on the medical staff, but the Spurs have a pretty good one, and you would think they can get this figured out and ameliorate chronic pain by this point.
I don't exactly with agree with Kawhi's decision, but,again, I see where he's coming from. He's been cleared, but it doesn't appear that he is or is going to be 100%. Unfortunately, this ordeal has caused a schism between Kawhi and the Spurs that might be too much to come back from. We'll see.