They do say, though, that the job is to "Design and implement major pieces of the salesforce.com core database infrastructure". That doesn't sound like they're talking about Heroku, at least not as the main reason for their hiring spree.
Perhaps the person writing the job ad was embellishing by calling it a major piece of the core database infrastructure.
[1] http://www.wired.com/wiredenterprise/2012/10/salesforce-orac...