Reversing a binary tree on a whiteboard is certainly a bad question to ask. But I would argue, for all intends and purposes still a miles better indicator about future potential than if someone knows how/why TLS work. Yeah you can read that in a book. I can google it. Useless for interviews.
If you are hiring for a position that requires you to implement TLS, sure go for it. But that is not the rule. And what are you going to do after he has implemented TLS? Will he be able to work on something completely different?