But rolling a standard datastructure (and what's the "standard library" for a red-black tree anyway - in Python, or in Java? - or C++ for that matter) proves that you can do something "tricky" and hopefully get it right. You can understand and reason about what you're doing. If they don't let you Google while doing a "CS" type problem, of course, then they're insane.

When it comes time to do something novel, that understanding will be very useful.

People pooh-pooh Google's hiring interviews, but when was the last time Gmail's search results on your email were as bad as Bing's (I use this example because it's not their general index, I'm asking about your mail.)

There are a lot of good reasons to bone up on CS 101 (and 201, 301, 401, and 501).... I disagree that it's just "studying for the test".

Even then, you'd have to know the characteristics of various data structures to be able to choose the right one.

On top of that, in almost all code beyond CRUD, you'd have to write some custom algorithms - there, the knowledge from data structures and algorithms is really helpful.