That isn't a whole lot in the grand scheme of things, wonder how they picked the videos. YouTube gets like 700k hours of videos uploaded a day, and a million hours is probably like 10bn tokens out of the 13tn for GPT4
english speaking rate: ~150 words per minute
gpt tokens per english word: ~1.3 tokens per word
1M hours = ~12B tokens
I'd fudge that up a bit because it's probably more tokens from non-English but comparatively seems like not much