Front brake first all well and good for pavements (I can see the logic but I have never had a problem using the rear but loading the front as supplementary power) and surfaces where grip is good but for MTB'ing it would seem Sheldon would agree with me:
When to Use The Rear Brake:
Skilled cyclists use the front brake alone probably 95% of the time, but there are instances when the rear brake is preferred:
Slippery surfaces. On good, dry pavement, unless leaning in a turn, it is impossible to skid the front wheel by braking. On slippery surfaces, however, it is possible. A front wheel skid almost always leads to a fall, so if there is a high risk of skidding, you're better off controlling your speed with the rear brake.
Bumpy surfaces. On rough surfaces, your wheels may actually bounce up into the air. If there is a chance of this, don't use the front brake. If you ride into a bump while applying the front brake, the bicycle will have a harder time mounting the bump. If you apply the front brake while the wheel is airborne, it will stop, and coming down on a stopped front wheel is a Very Bad Thing.
Slippery and bumpy surfaces would cover about 95% of the terrain we see off road so it would seem strange that this MTB video would teach front brake first all the time.