I have no doubt that the number of people capable of running a sub 5 is less than the number of people capable of running sub 3. The question is what are the actual numbers. Taking my HS (from 40 years ago), we had 5 sub minute milers in my graduating class. Of those, only 1 went on to run a marathon under 3 (the others never tried AFAIK). How many other people from my class ran a marathon under 3. I doubt any, but lets say there is 1 or 2.
If I expand the same to the entire 3 years I was in HS, I'm guessing there was 12-15 runners who ran sub-5 for that entire 5 classes. I can add 3 that I know went on to run sub 3. There's probably 1 or 2 I don't know of. But still, the total number is smaller than the number who ran sub 5.
If I expand to my college years, then it's the same. And it's not as if the marathon was not a big deal in my HS years. It was.
I personally know very few sub 3 hour marathoners who did not at one point break 5 in the mile. I know lots of sub 5 guys who never ran a marathon (even though they were capable of sub 3).
A sub 3 marathon may take less talent than sub 5 mile, but it takes more training and more grit. So I'm going with there being more people who have run sub 5.