I would say non-recruited athletics are “just” ECs, but ECs are potentially very important.
As I understand it, when holistic review residential colleges consider ECs, their top question is usually whether this applicant is likely to do this activity, or similar, at their college in a meaningful way. This can be part of their admissions mandate, to enroll students to get in the pipelines for all sorts of valued student activities.
Depending on the activity, they may be looking for different things to indicate likelihood of being a meaningful contributor in college. For most activities, sustained dedication of time is likely valuable evidence. But some activities may also require certain demonstrated talent/skill levels to be considered useful, if that is what is required to be a meaningful contributor.
OK, then non-recruited athletes can do a variety of meaningful activities. Like some sports don’t do recruiting, or not at the top level at that college, but are still valued. This can include club sports, again including sports where club is the highest level in general.
OK, so realistically, a lot of people who just participate in HS sports may not actually be good bets for the more competitive club sports in college. Indeed, some club teams have players who might have been recruitable elsewhere, but didn’t want to go to any of those colleges. Others might just want to back down the commitment level. Others might be below recruitable levels, but not much. Others might be great athletes who want to switch sports in college. And so on.
OK, so being very dedicated to, and accomplished in, your HS sport, or sports, may look good as a potential college activity even if you are not recruited at that college. But many other things might serve a similar role.
So this is one potentially valuable type of EC, but only one of many.