Have a look at the interior which is standard E39 M5. They did not squeeze M5 mechanicals into an E9 but if they had, I agree that it would have been spectacular.
Their method was to "peel" the skin off an M5 and replace it with the skin of an E9. The results look great but a deeper look shows that the proportions are just so slightly out.
Another clue is under the bonnet/hood. Thats a standard E39 engine bay.
I would bet that it goes like the wind, handles superbly, and breathes life into and old M5. Whether that 'life' is worth what they are asking is debatable.