first of all, this is not from DragonBall Heroes, it's from Episode of Bardock, and anime adaptation of the manga of the same name (by Ooishi Naho, not Toriyama) for the 2011 Jump Festa.

second, no announcement has been made as to whether this is 'canon' or not in terms bardock being the real legendary super saiyan or for that matter whether this even happened. as the fine folks over at daizex.com pointed out in an episode of their podcast, the most coherent timeline of the saiyan race has them actually conquering planet vegeta (and potentially even arriving there) within 50 or so years of the show starting. if that's the case, it makes it tough to imagine that bardock was there thousands of years ago or how he could be the LSSJ, notwithstanding all of the questions of training, etc.

on the continuum of ssj transformation experiences though, i'd rank this one pretty high. goku obviously would be my number 1, but after vegeta (and maybe future trunks) we don't even really see anybody do it for the first time. there's a glimpse of gohan but not too much of his ssj1 transformation (perhaps to add to the drama of ssj2), and other characters just kinda do it. Bardock's transformation made sense on the emotional level the same way that goku's did: no way out, hopeless situation, friend being hurt / killed etc.