I can kinda of agree that Strava weeds out a lot of non-serious riders, as well as really serious riders, who would rather just ride for fun, or track metrics with a better system.

I also think Strava allows you to set your segments as public and private so when you ride you can compare or keep secret your times. From this i would say that a lot of the people making there times public are people who think they are quick (some probably are) and the slow people probably don't want to be last so they keep it private.

So if you look at this as a whole they greats and casual/don't know about strava/don't care about tracking get chopped and your left with middle class riders who use strava, and inside that you get a lot of people not sharing times which i would think is because the times aren't competitive, leaving only the faster strava users.

That's how i would look at this anyway, there is a good chance I am wrong.

EDIT: I just thought of this as well, segments don't need to be ridden as a whole ride either, so you can have guys who show up and ride only that segment for the specifics of getting a good time on the board, and then you can have guys who have that segment as part of a whole 100km ride. That would really skew times and speeds too.