I'd like to see the study also but I'm not really skeptical. I've previously seen one showing that the effectiveness of high intensity interval training varied according to individuals (sorry, no link there also).
If so, it means that we can be better to do"what works" individually than to slavishly follow a formal training regimen. Maybe. Assuming we can actually test "what works" in some disciplined and precise manner. I may suffer from some "confirmation bias" since I don't like structured training and would rather ride harder or ride longer as the whim strikes me.
I think OP is already doing well since a couple of centuries at 15 mph is nothing to scoff at. I've personally had the same questions and I personally think there's no accurate answer to "how much better" is possible. The first couple of years with rapid improvement, it's easy to feel that you're stronger, almost on a day to day basis. After working through a plateau the gains are more incremental. Can we keep improving, but less and less, asymptotically approaching some line, where does that line move with respect to age? Beats the heck out of me, but I suspect that that also is a very individual thing.