Errr.. The point is we don't actually know the specs of the videos edited, aside from the resolution.
It should also be noted that Kaby Lake's new hardware video support probably applies to none of these videos, as there is currently no support for it in macOS. The support is only available in High Sierra.
So the 1st 4K video is from a Galaxy S6, probably 2160p30 h.264. So hardly surprising. Indeed, the guys said the 2016 MacBook handled 4K fine too, which I assume is the same type of 2160p30 h.264.