I don't think there was a misdiagnosis. If there was, it's not a big deal - bone spurs can be hard to detect. But more probably KG has that bone spur for years, but only now it's started affecting him.
I've said these multiple times and I'll repeat it again: it's very probable surgery wasn't an option.
A bone spur rubbing on a tendon in the knee can be a lot of things. The current information is extremely vague. What’s the size and type of the bone spur? Which tendon are we talking about? Is there any ossification going on?
Generally, when a tendon is affected, the procedure is to avoid surgery at all costs – because of the risks of weakening the tendon. The surgery itself is not necessarily complicated, but the healing can be. It’s basically the same of recovering from a ruptured tendon – if it’s the patellar (something that wouldn’t surprise me, as KG was clearly suffering from tendinitis, it would be the same process Antonio McDyess went through.
Anyway, generally you try the conservative treatment – rest, strengthening, massages, etc, and only as a last resource surgery is an option.
For now, all we can do is to wait and see – with the limited info available there’s no way one can speculate about the probability of a surgery (although it becomes more probable as the time passes). That’s basically what the doctors are doing as well, although they have more information than us – wait and see.