Danny MacAskill of Inspired Bicycles... got skill

Wow, I loved the bit with the tree. It made me laugh.

In some shots it looks like he can pedal backwards, but he also has a freewheel. Does such a hub exist?

Yes, it's called a freewheel. Try pushing your freewheel bike backwards, the pedals will turn backwards as they only freewheel in one direction. After a 180 he has to let the pedals turn backwards to allow the momentum to carry the bike backwards, otherwise it would just stop. It's riding fakie, bmx and mtbs have been doing it for years.

regarding the hubs, the ratchet mechanism of some hubs, dis-engages when going backwards.
it's called a freecoaster hub.

These do exist, but obviously he doesn't have one, otherwise he wouldn't need to pedal backwards, he could just freewheel.