I'd like to address a few things.
First of all, the video is a compilation of clips, and is filmed across a number of different creeks. It is not just one river or system being featured. If you recognize a clip or two, that's great, enjoy the fishing and don't fret - it's incredibly unlikely that most other people will have any clue of the location(s). They are not popular rivers by any means; I can say that after I spent 10 full days this year hiking my 4 favorites to only see 2 sets of footprints on a single one of those systems (the "busy" one).
I've spent probably close to 100 hours in the last 3 years exploring upwards of 120 km of different watersheds that would "likely" hold these large cutthroat. A handful of times have been complete busts, but other times you run into magical conditions. It's very time specific, and in a number of rivers, it's also very water dependent. It's a lot of work that the average angler isn't willing to commit, and often times simply can't time right based on scheduling and when rains occur.
As for the spawning comments - nearly every fish that sport anglers target in rivers in B.C. are in the rivers to spawn, and are migrating or staging to do so. The reason we encounter those large schools occasionally is because I specifically focus my efforts on the lower 1-4 km of each stream after a water event to intercept moving fish. Go back 2 days later, and where you found that school of 30-120 fish now may only contain one, and the others often seemingly vanish. Cutthroat are notorious for exceptionally short residency times, fast movements, and for spawning on high water events. In short though, we do not target spawning fish, and make every reasonable effort to not even encounter them. In the case of those big colored bucks, I would personally love to see photos of some bright silver ones that you guys have encountered. I've seen them looking like that (colored and kyped up) out in the salt and in large lakes in the summer - cutthroat often don't lose their sexual dimorphism/ secondary spawning characteristics after their first spawn.