Hey SUP DOC, quite a few of us in Sydney still MTB - Goatie, Jedjunglesnow and myself.
DTM used to build the gnarliest 'northshore' style drops around here until he absolutely creamed himself. Thats pretty much when he and Goatie picked up SUP.
I still ride whenever I can for overall fitness. I recently signed up for the the 'single track mind', series but had to pull out due to the left wing being out of action.
Couldn't agree more - there is no better combo for overall fitness and clearing the mind going from ocean to bush.
As you say these videos are the extreme.
There are so many awesome, smooth single tracks around here from Menai, to Ourimbah and Awaba where you can fang around at mach speed and not leave gthe ground - just watch those tree's!

Just like SUP you don't need to backdoor pipe or Back-bombies to have fun!