It's obvious that they're not going to use the code you wrote at an interview in production. So "just use a library" is not a relevant answer. The question was probably posed to give you an interesting engineering challenge to work through with the interviewer so that they could see your problem solving process. Instead you just argued with the interviewer.
The GP's point (which I supported) is that it's a waste of time to take a candidate through scenarios that are not at least similar to what they'd actually encounter doing the actual job.
But the point is that if you cant do something so simple, like traverse a tree, then you probably don't understand how trees work to begin with.
Similarly, I've never had to implement fizz buzz for real, actual, work. But I wouldn't trust a programmer who was incapable of doing fizz buzz without importing a fizz buzz library.
But the earlier comment wasn't talking about _traversing_ a tree, it was talking about _balancing_ it, which is a much harder problem and one I wouldn't want to do without google either.
