Yes, that is right, except actually they rev the Model S (and I’m sure other models) all the time, a recent example being the new generation of center display.
As far as I know, it’s just “the year in which the car was built” (or maybe delivered), nothing more than that. Every now and then this is slightly problematic, e.g. there’s different roof rack mounting hardware for Model S built summer 2015 and later than there is for ones built earlier, so “2015 Model S” isn’t enough info. But mostly, I like the practice.