I see the same thing with the kids around me! The speed at which they absorb historical details from videos is impressive - like, they know stuff about palace intrigues or forgotten figures that I didn't even know about. The plus side is that history becomes less boring and more accessible - a good creator can make something fascinating that looks deadly dull in textbooks. The problem is that these videos inevitably simplify things, cut out context, build engaging narratives that aren't always balanced.
What happens is they pick up facts scattered here and there, without the conceptual framework you get from school. Like, your cousin might know details about a specific battle but have no idea what caused it, what the economic consequences were, how it connects to the rest of the period. Videos are short and need to hold attention - there's no time for nuance, for saying "this historian thinks one way, that one thinks differently." And then there's the risk that the creator just gets it wrong or sensationalizes everything for engagement, with nobody to correct them.
It's not all bad or all bad - the point is that school should integrate with this stuff, not compete with it. If teachers pushed them to track down the sources behind those videos, to compare different versions, that would be a powerful mix. Unfortunately, a lot of schools still don't know how to deal with it, and kids end up learning on their own without a critical eye. What really worries me is that history becomes pure entertainment, when it should also teach you to think critically about how information gets served to you.