Although this wouldn't apply to ancient humans, I have heard the hypothesis that many people in olden times didn't have as many problems with cavities because many people drank well water. And well water has naturally occurring fluoride.
To respond to a sibling comment: the relative levels have been looked at, in detail, and existing policy reflects what we've learned from that. Scaremongering over it influences real negative health outcomes, particularly amongst those with the most limited access to comprehensive dental care. Flouridation ain't quite as big as say sanitation, or antibiotics, but it's still up there on the list of biggest public health wins ever. By all means investigate it critically, but perhaps in a way more sophisticated than "have they looked at it in more depth than me spending 10 seconds googling?" imo.
This typically happens with teeth when young children drink fluoridated water, and also drink formula, which has fluoride, mixed with municipal fluoridated water. Or if the well water of an area has too much fluoride.
Do tribal people around the world living traditionally have cavities or not? What is the quality of their teeth?
They wear jeans, though. So they might be accessing nearby cities and the Youtuber avoided to mention that for clicks.
However, when I left my childhood home to attend university (and moved into the city afterwards), I haven't had a problem since. Then again, this is just one anecdote.