The times he treated him had nothing to do with his eyes up to that point. McCoy was learning on the fly.
Very much my point, if he were an expert, if he could justify being Spocks regular physician (as opposed to finding himself muddling through ad hoc) he shouldn't have to learn on the fly, he would already have done his homework and known this stuff ahead of time.
After all it's not as though he didn't know he would be responsible for a Vulcan, he was assigned to a ship which had one as the first officer, he was responsible not only for treating him but also whatever regular healthchecks he would undergo.