I believe the 500 figure represents the average attendance across all teams in 2019. Speedway doesn’t officially release attendance data, so it’s just their estimation. According to their statement, they estimate around 700 spectators on average, with a peak of 1500 during the playoff finals.
I think that figure is low, but then that’s how they justify the requirement of other events for stadium viability. I agree with you that fans of all clubs should support this application, however cynical they are of it, as it’s the only chance of Speedway returning to the Swindon area.