Part of the interview is an intelligence/memory/experience test. They don't want to use rigid requirements ("must absolutely have five years experience") and it would probably be illegal to administer an IQ test (and the sort of intelligence IQ measures isn't always the same as the sort of intelligence needed for the job).

In addition their culture seems to be that once you're on the job (as a salaried employee vs. a contractor) they'll do all in their powers to ensure you aren't laid off, get fired or quit out of boredom: you can freely move from team to team, you generally work flexible hours and with little micro management. The tough hiring (with far too many false negatives but no false positive) requirement is why they're able to mantain such a culture.