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.
Be that as it may a Salesforce.com representative told me that the hiring is related to the Heroku Postgres service, not a migration from Oracle to Postgres [1].
Perhaps the person writing the job ad was embellishing by calling it a major piece of the core database infrastructure.
