Olympic Auditorium in Los Angeles isn't that big.....it wasn't an actual concert and the filming was specifically for the video. You don't actually see alot of people. You don't actually see people singing along. At most you see some panning close ups of the audience or pounding fists..but all close ups..you never get a camera sweeping across the entire Auditorium showing it filled with people.

But yes, they did have a fan base...it's not like they were unknown or had to beg people to go see them. Radio announcing the filming of a video with the chance to be in it helps.