They'd have to be qualified enough to get an actual offer. He didn't say anyone interviewing would get the $500. Not sure what sort of gaming could be done with that criteria. And with that carrot for qualified people, he's likely to get more applicants than otherwise, which is the point.
Well, not every developer is making a ton of cash, and a fair few qualified ones may be quite happy to do an interview every day or so for a bit of extra cash.
Heck, if enough companies offered $500 for a successful interview, some people would probably end up making more from going to interviews than they would accepting the offer.