This seems like the critical premise to the argument in the article:
There is no reason to think that physicians who take gap years will retire at an older age than other physicians, thus every gap year will shorten a physician’s working years by 1 year.
I would not necessarily be inclined to make assumptions either way. Like, I definitely would not assert I know they will all work to an older age as physicians, but I am not at all sure they will all work to the same age. I actually suspect this would get even more complicated if you really tried to model it, as of course not all people who go to med school actually work as physicians, or not their whole careers, and I am not confident that taking gap years would not interact with those variables as well.
Like this is just speculation, but is it possible that gap years marginally reduce the number of people who rush into med school, end up not liking the profession, and end up not being physicians for long or at all? I don’t know, but it could be, right?
That said, even assuming this is true–if there is a shortage of physicians, isn’t the real answer that we need to be producing [edit: and retaining! see below] more physicians? I get that is a complex subject, but to me, complaining about the speculative marginal effect of gap years seems to really be dodging that central issue.