Edit:  I created a segment on a ride I do with some regularity that includes an out-and-back in the middle of it.  I waited a while and when I checked, it had populated with many (all?) of my previous rides so they qualified.  IDK if there were a few that didn’t.  Next time I do that segment I’ll try cutting the out-and-back short by ~100 yards and see if it still qualifies. 

There is a margin of error Strava allows for GPS drift and other inaccuracies around segments. If it’s within ~10m or so it will be counted. If your out and back bit is rather long and you skip it, the containing segment will probably not get counted. 

Yep.  Garmin Edge units used to have trouble with out and backs since the outbound and return track points are very close and can even overly each other.  It would sometimes think I was on the way back when still out outbound. On a segment with an out and back, I could turn around prematurely and never be more than 10m from a track point. Let’s see if Strava “keeps track” and sees that i skipped some.  I expect it will, but proof is in the pudding.