That makes some sense - but really after the 3 previous tryouts - already having gone through skills, six on six etc. - coaches have a decent sense of who can play and really do need to see how kids run full field. Maybe some more time in smaller groups would have helped but full field scrimmage tells you a lot as well.
Only way to tell if team was already picked will be to see if new players join and others who played last year don't make it.
I'm not a fan of the full field considering how many kids there was. But for a small handful of studs, hard to get seen. For the 21s it was readily apparent that one field was the kids they were considering and one was not.