Unless you need a ticket to attend the outside portion of the events around the stadium, the organizers can say whatever they want regarding attendance numbers. 300,000, 400,000, 500,000...or even a million.
Being outside the stadium is really just for the casual observer. Hang out with friends, a couple cocktails, see your teams 1st pick, and get the hell out.
Anyone who follows the entire draft would probably be best served being at home. TV, computer, phone, draft publication, food & beverage....what more can you ask for.