Is that the AX8 on top of the FXII?
Well he was pretty clear about it being software so I'd assume that like others have said that the AxeFXII was just being used as the audio interface.
Its all about the algorithms. Axe FX and AX8 are hardware AND software. He might be working on the software end of it: Putting together blocks on the FX II that can be ported over to the AX8 to make presets when the firmware is dialed in.
Or it could be the guitar signal is being routed to a variety of devices so comparisons can be made: and the Axe FX is one of them.
Another possibility might be ...
David makes everything sound good. Really versatile player. I've been using some of his MapTracks to get out of my position playing ruts. Good stuff.David's video sounds good.