Since all helicopters fly in a vortex ring (the trick being not to be at the epicentre of that ring) there is no guarantee that a drone would have been blown away. Even if a drone approaches below the level of the helicopter, the ring causes anything close enough to be recirculated. Just watch videos of helicopters landing on a dusty or sandy field.
Having said that, there is always the possibility that the instructor forgot about the dangers of "vortex ring state" especially on uneven ground (as referred to in the article) and ended up not going where he expected. Much easier to blame a drone than say you forgot about the intrinsic danger of moving slowly when near the ground.
Interestingly if you search for vortex ring crash, you get just as many DJI Phantoms shown crashing due to "vortex ring state" (not continuously moving out of the vortex) as you do helicopters. Videos that do show choppers crashing due to vortex ring often show them landing heavily on the rear end of the skids before toppling.