That doesn't sound right? I thought it opened 4 hours before?
I found this online, but didn't find anything official:
Tailgating is permitted in designated lots for a maximum of
four (4) hours. Guests can begin tailgating four (4) hours prior to kickoff; however, tailgating becomes prohibited at kickoff. Guests looking to tailgate on Allegiant Stadium-operated property are required to possess a valid parking pass.
Per UNLV tickets, it is 4 hours, maybe they person you talked to was providing bad information?
www.unlvtickets.com