I would say that datasheet may be very precise, and still even correct numbers are often misinterpreted. Customers are tended to remember biggest number outside of context, and expect miracles, which not happen.
So such customers are tended to
expect that all 500 users will perform on theoretical max rate on any
distance, which of cause never happens. No technical person would expect it, of cause.
280 km/hour on your new car speedometer doesn't mean that you will be driving with such speed all the time, even so number is correct, and properly represents that you have a really fast car, much faster than one with speedometer limit 160 km/hour.
Support for 500 associations is actually very useful in such environments as public events, hotels, malls and so on, as a big part of the clients are smartphones in the pocket of people passing by, not creating big traffic (if any at all). With many vendors (including 2 of 3 other WLAN grands) in this situation association tables are filling fast and real clients may not have access to the network.
Anyway, this number properly represents that Ruckus handles such situations better than most other vendors, which is a real fact.