I suppose it would teach them the bare minimum to where they'd be at a teachable state on the job and potentially contributing soon thereafter, but yeah, it has to be pretty rough for both parties.
That said, I'm suspicious of this too. You can certainly teach someone git, HTML, CSS, JavaScript, SQL, and Rails in 12 weeks - but you can't go very deep with any of them.
But if the company decided that they needed a new component that would be a standalone Java service (or whatever), I a) wouldn't feel confident that the person would be able to pick up Java in a reasonable amount of time, and b) wouldn't trust them to structure the application in a good way, as they just don't have the experience building real things.