evidence 1: besides ruby, only s1 contestants have been seen in these videos, with the very first videos not counting, as they only serve as music. this explains why when four and x first appear in bfb 1, none of the s1 contestants wonder about it (i know coiny reacts to it, but it never shows him not knowing what it is, and just asking if four could bring pin back).

evidence 2: in the latest subscriber milestone video, we see 6 and 9 on a bike going to meet 15. on the way there, we see what is supposedly yoyle city, even though we know it was abandoned sometime before bfdia, or even in between.

quick rant: when 6 and 9 are on the bike, i think which destination they go to is out of order, as we see them in the bfdi land, then suddenly at the broken cliff thing (i forgot it's name) and then in the land of tpot, possibly suggesting that it took over 10 years for them to finally reach 15, with it starting at the equation playground, and then ending at the swamp, with space being an accidental route while heading there.

evidence 3: in one of the sub count videos, we see x and seven going to learn some things about equation, suggesting that that sub count video took place sometime/after/before x.f.o.h.v, furthermore suggesting that it took place sometime before bfdi.