I wouldn’t hire anyone who couldn’t reverse a binary tree (with a bit of coaching is fine too). I can certainly understand why someone without white board experience would initially freak out and assume it’s difficult but I’m confident I could ease some panic and point them in a reasonable direction.

We’re talking like 4 loc here…

But there are absolutely people with the title of senior engineer who cannot understand the concept of swapping some pointers and recursion.

Of course you wouldn't. But with a certification exam that included such things, you wouldn't have to ask.

And more importantly, like I said, you aren't choosing their salary and job level based on those skills. You're choosing it based on their experience.