im a big convert to using your front and back foot equally on 3flips, but that said i don't think he's gone all the way to underflip.
if you watch the way mullen does 360 underflips, you can see even without looking at his feet that the flip has a distinctly different looking rotation (you tend to 'see' the flip more as it's own motion than just part of the parcel of the 360). on felipe's the way the board itself flips, looks like a classic tre.
(i know it's out of kaspar so a bit different but 3.14 is a good example of what i mean)