So in short, take a 50mm f/1.8, convert it down to a 35mm f/1.2, stick it on a crop sensor and the crop factor brings it back to a 50mm f/1.8 FF-Equiv.
Not as nice as a 'real' FF Mirrorless, but hey, the physics works and as long as the glass they're using is decent quality and not just jam-jars like you find on FD->EF adapters, then why not?
Also note, there's absolutely nothing (except maybe legal/patents?) stopping them from making this for EF->EF-M, or from any other slr to any other mirrorless.
Actually, there's nothing stopping them from making this for MF->EF either.
Given how huge the MF image circle is compared to FF, you can get a 1/1.6x crop from a 645 lens. Say, take a Contax 645 Zeiss Planar 80mm f/2.0, and convert it to an extremely sharp 50mm f/1.25. Or mamiya 80/1.9 goes to a 50mm f/1.19.
6x6 lenses like Hassy can go even further, like that very very nice 110mm f/2.0, with a 1/1.8x crop factor, with a speed boost to FF becomes a 61mm f/1.1. Even if you have to drop $500 on the adapter, and $500 on the lens, that's still half the price of an EF 85/1.2L or 50/1.2L.
(of course, you could always just shoot the same lens on film, or buy a a Mamiya 645AFD for $500, but then a Leaf/Phase One back is $2k for 20MP or so very used. Shooting that on a 5D3 with uber-iso capability would be great)