No, some people are just bad communicators in particular mediums, and some mediums are bad channels for conveying other ideas.

Fundamentally, not every bit of knowledge is easily translated into words (or images). You see it a lot when teaching others how to cook (or especially bake), where smell, texture, feel, and all those are both important and knowable, while simultaneously difficult to describe. I can show people how to bake a sourdough loaf, but reducing it to text loses a lot, to the point where the typical person won’t be able to actually derive the knowledge from that text. And plenty of people I’ve tried to teach don’t have the attention to detail to be able to absorb it. I can be an expert in the actual craft while not quite grasping why other people in my orbit just don’t get it. That’s the phenomenon of superstar athletes retiring and then struggling to become decent coaches.

The experts in a lot of fields didn’t learn their knowledge in a book. Or even instructional videos. Limiting your definition of “knowledge” or “expertise” to only be the subjects that can be learned in those settings is too small a view.

No amount of book reading will teach someone how to be a good basketball player, a good guitar player, a good public speaker, a good friend, or even a good writer. That doesn’t invalidate their expertise, or even require they be good at explaining their craft to be considered knowledgeable in those fields.

At the end of the day, plenty of people are bad at communication. But just because someone is bad at communication doesn’t mean that they’re inherently not knowledgeable. And that’s the fundamental error in your view.