From their web page, they only make money if there is a hire:

http://developerauction.com/for_employers "If you end up hiring a candidate within 6 months of viewing their profile, we will invoice you..."

Note the phrasing "142 startups... submitted over $30m in job offers." Sounds like if company A bids $200K for SuperNerd, and company B bids $250K for SuperNerd, they'd count that as $450K of submitted bids. SuperNerd may have declined both offers, so income might be zero. See the "Vanity Metrics" section of this article (but ignore its title):

http://nickoneill.com/silicon-valley-is-filled-with-liars/