Of course he should have, but do you have any idea at all why he did not?
I don't know exactly how his brakes failed, and have never had to ditch a truck myself, but this can happen two ways: The brakes will kinda feel like they're gradually failing to the driver, or they'll 'abruptly' fail. A loaded truck going down a steep grade will pick up speed like you couldn't imagine. You'd have to actually drive one to understand just how brutal gravity can work.
I'm guessing he thought he could maybe 'ride it out', and would be able to slow down at the bottom of the hill. Or he simply was too stunned to make the decision to use the runaway. When you're flying down a hill with no brakes, these would be split second decisions that truck drivers are not trained for. Pilots get trained for this shit. Truckers do not.
We are expected to 'learn as we go' in this industry, which often doesn't work out well.