I've never driven artic, but to be honest good driver should be always aware what's behind him. I can't see on straight motorway that massive blindspot behind a trailer being any bigger than in normal rigid lorry or a bus.
Besides, I never advocated heavy braking.
It's just hard to say from the video, if there was a heavy braking needed to avoid undertaking by this truck driver.
In the case of some fool doing something stupid, like in the video where the lane hogger decides they are going to pull in on the central divider, the safest thing the truck driver can do is keep going at a stead pace and keep out of the way. It dose not mean he was not breaking a law but it was the safest thing he could have done.
Oh and he was most likely talking to himself!!!