That's not true. I downloaded ML with the code for my MBP Retina, which hasn't arrived yet and installed it on my Mac Pro without any problems.
Glad it isn't true, really, because if the app store / installer could do that then why have people enter anything in a form at all, they could just grab my serial, check the purchase date with Apple, and go on with it.
That's what I kind of expected to happen actually - a dialog saying "Are you using your new retina MacBook Pro? If so, you might be eligible for a free upgrade to ML. Do you mind if I check this with Apple's servers? (Y/N)".