Matches played before time starts?
From the player's perspective, the announcements weren't clear enough, or they'd have known not to start; granted, they share some responsibility here, too - you'd hope they'd notice that no one around them was playing yet, and often when this happens, a neighboring match might speak up.
Still… the reason we don't want players to start their match before the clock begins is to ensure everyone has the same time constraints; it's not fair for one match to get 55 minutes - esp. if they use it! - while all the others only have 50.
In this case? They didn't benefit from starting early, other than having more time to relax before the next round. And, it's a prerelease - fun is meant to be had! - so really, no harm done. Ask them to be more attentive, and leave the result as reported. (i.e., I would not cancel it in this case)
At at Comp REL event, if you notice someone starting before the round begins, just ask them to pause and wait for the official announcement. It's not good customer service to cancel decisions like starting hands and mulligans.
d:^D