Salesforce spends a huge amount of money on licensing their horizontally scaled Oracle databases. I'd venture to guess this signals Benioff exploring a strategic bet to tell Ellison to fuck off once and for all.
The posting says they're hiring only 5 now, 40-50 next year;I inferred they're still in an exploratory phase because if they were all in, they'd be hiring more than 5 now. The posting doesn't say they're doing a migration but having worked on Oracle -> PostgreSQL migrations and sharded database infrastructures, it makes sense.
Of course nothing is ever final, but publicly announcing something like this is already sort of a "fuck you" to Oracle, so I'd be surprised if they weren't a 100% sure of their plans to switch.
Having worked at Salesforce I can say they take _forever_ to move on anything. When I left the company two years ago they were just starting to talk about Postgresql as a option to replace Oracle..