Bad example really. There are many companies where you don't need to know this stuff, Google is not one of them. You want that 350k a year, but you don't know the basics about computer science to find the k-th highest element in a BST, well doh. What can I say...

To be honest, this is really a super simple question and I would be stunned if this was anything but a warmup for you, like the interviewer giving you a simple question to get you into focus. I haven't done anything with BST in years but still I could easily do this with a piece of paper. Back in the days, Google was asking to insert an element in a Red-Black-Tree. Well, this is a clusterfuck and far too specialized. But questions like those were rightfully banned.

Yeah sure, it's not what you do all day, but not being able to answer these questions has implications. There are many code monkey mills where you just write some JS code to hack a webpage together and it would seriously bother me if they would ask you such questions.

Just rethink what you really want. FAANG is not for everyone.