Before Covid, I was 100% a couch potato. After Covid hit, I dragged my hand-me-down bike out of the garage and started cycling, just to get out of the house. A few months later when we started returning to the office, I just rode my bike to work instead of driving, and I haven't stopped since.
In the office where I work, there are two competitive triathletes that I know of. Neither of them bike to work. One of them kept telling me how she wishes she could bike to work but the distance and time doesn't allow it. My own brother is a very fit rock climber and lifelong cyclist and he doesn't bike to work because he lives too far away. I know a number of co-workers who are regularly do running, weight training or basketball, and none of them bike to work. Most of the bike commuters I see on the road on a daily basis, tend to look more 'dorky' than 'fitness/spandex'. No different than how it was in high school- the cool kids had cars (or had friends who had cars), while the dorky kids tied their pant cuffs with rubber bands and rode their ten-speeds. So I don't really buy the self-selection argument.