Ok, sure, if someone doesn’t know the basics, then they can’t be an expert in that thing. But that hasn’t been what we’ve been talking about in this thread, and your initial comment was that someone can only be said to understand a thing if they can explain how it works.

And I’m pointing out that:

• Some people are bad at communication, and understand things they can’t explain, because their explanatory ability is hindered by their communication skills.
• Some specific concepts are not easily reduced to words, so they are inherently difficult to explain. That doesn’t mean they can’t be understood, or that nobody understands them.

Dropping back to only talking about the basics kinda ignores huge swaths of human knowledge and understanding.