If you already know the algorithms, sure. The thing about reversing a binary tree is that no one already knows the algorithm. It's not something you'll find in any textbook. It has no intrinsic meaning, so why would it be there?
That makes it far better for an interview question. It's just a test of how you handle the sort of nonsense task that makes up so much day to day work. "Why do I have to frobnicate these doodads?" "Because the doodad processor requires it. That's just how it works, so we need to do it."
If you're doing matrix multiplication from scratch it sure isn't!
But I do think that by hand implementations of k means or k-nn or cross validation would be appropriate for folks who are interviewing for ML positions...
That makes it far better for an interview question. It's just a test of how you handle the sort of nonsense task that makes up so much day to day work. "Why do I have to frobnicate these doodads?" "Because the doodad processor requires it. That's just how it works, so we need to do it."