I agree that hitting a tar snake is disconcerting but it you've ever done any dirt riding, the feeling of the bike wiggling around is normal to you. Unless you're at the very edge of your tire traction, it's a non event. The tire will hook right back up once it gets to the other side of the tar snake. Guess he had to "lay 'er down".
I've hit tar snakes in the middle of curves. It's caused some puckering, but you're right in that it hooks right back up once the tire slides off the snake.
While I hear you on the "had to lay 'er down" thing, I think it might be slightly unfair to malign the guy in this case. In this case, the guy would have been completely upright, not leaned into a curve. It's not generally reasonable to expect the bike to start feeling like it's gonna come out from under you when you're bopping down the road in a straight line at 50-55 mph.
Although it's pure speculation, what I think might have happened is this:
The guy is in the far-right line position, staggered correctly behind his buddy in front of him, probably doing 55 mph (the posted limit) or so. He hits the straight, 30-ft long tar snake, which was quite close to the white line. That causes the bike to unexpectedly drift right because of the normal crown/slant of the road surface. He panics/target-fixates on the ditch and, of course, ends up right in that very ditch.
Like I said, complete speculation. But it seems feasible.