A fully loaded tractor trailer had no time at all to slow down on that hill, actually. Braking hard would have resulted in a 3 vehicle crash, imo.
I agree a hard brake would have made it worse, but again, that's my point, there was no need for hard braking if he just starts slowing down prior to that (my assumption is that from his viewpoint - which will look differently through a camera - it appeared that the other truck was behind the plow and not trying to pass it). I'm quite well aware how trucks handle/brake. Reaction time is everything as it helps the driver make a good decision instead of a poor one like slamming on the brakes.
He actually quite clearly does slow down noticeably when he gets quite a bit closer to the plow (~9 or 10 seconds). If he'd have started that ~4 seconds before then, they likely would have been able to maneuver enough to avoid what happened (it would have given the other truck enough time to back off and get behind the plow leaving the lane open). As it is the other truck actually did maneuver quite a bit and avoid a major collision (a comment allegedly from his wife claims he didn't even hit the other truck, which is likely due to him slowing down even as much as he did before going to the guard rail).
Like I said, he's not at fault, I think he deserves credit for slowing down and taking the guard rail (knowing that it could have sent him over it). I just think a bit better reaction to the situation could have improved the outcome. Personally I think semis should have technology to help drivers handle their limitations.