I know some versions of it, like Feynman saying that if he can't explain it to a freshman, he doesn't understand it.

Likewise, if you write code that only you and someone smarter than you can understand, you don't understand it.

That does bring a risk though. If you write simple code (e.g. in Go) that does what it does without any trickery, a more inexperienced job interviewer may look at it and scoff because you're OBVIOUSLY not a very good developer if you stick to simple code.

But that's a good litmus test, if they're like that, run away. They don't write code to solve a problem, they write code to flex and impress themselves, or to provide them with job security, or CV boosts.