Comment by RetroTechie

Maybe not so thin: much humans' knowledge is embedded in things we create (outside of language).

For example the design of a machine may have it tolerate inputs way outside spec & work fine. It may be built to take a beating, while no manual mentions using it in a rough environment. There may be subtle or not-so-subtle tweaks done to it over the years.

So that machine embodies knowledge, that may be 're-discovered' (by observing machine in action) long after its original designer is gone.

Another example: the design of traffic systems, the layout of cities (mostly organic growth), and how it affects the flow of people & goods through that city.

That's just a few examples. In short: knowledge is stored in other ways besides books/videos etc, or people's heads.