I doubt that even as great a rider as Burt was consciously trying to lay it down. Nearly all the time, it's simply what happens when controlled but desperate braking goes a little out of control and the rear end decides to shift around a bit to see what the front end is doing, and you're lucky enough to be in a position to do a lowside dismount at that moment rather than a highside.
For that matter, it was sheer good fortune that Burt and his pillion didn't
also bounce off the train. Having seen videos of actual highway incidents where folks dismounted this way, it never
looked intentional to me. (The only times I've ever seen it look intentional is with lightweight dirt bikes.) But more to my point re: Burt's good fortune, the riders I saw were never very far behind the bike when both came to a halt--with the exception of cases where the bike encountered something else on the road and flipped airborne for a while, of course. Otherwise, a rider in full gear only has a little more traction than a sliding bike; getting clear of the machine in any controlled fashion while sliding is not always an option, even in a case where you might want to. (And of course, all situations are going to vary in that regard anyway.)
Overall, if laying a full-size street bike down really was (a) something you could do at will, as the name suggests, and (b) had sufficient survival value, I have to believe it would be taught as part of safety courses--although, granted, it is a rather expensive method to practice repeatedly on pavement.