How is disclosure of previously undiagnosed long term health condition likely to be viewed when evaluating academic track record?
I would suggest that you don't raise the issue, for a couple of reasons.
First is that you don't know how it will be read. Some people might see it as an invalid excuse, rather than a valid one. Others might wonder whether it will affect your work in future. It is impossible to say. Saying such things won't, in particular, give you an advantage.
Second, it is natural for a person's productivity to increase over time and natural for an application reader to weigh the more recent work more heavily then the older work. That effect can occur even for someone with no health issues. I doubt that the change after the date of diagnosis would be noted by a typical reader unless you point it out.
This is really opinion, of course. If you are healthy now, focus on how you will continue productivity going into the future.