How prevalent are traditional CS questions in data science interviews?
It depends on whether you're going to write production code or even pseudo-code that will be implemented by others. If so, yes you need computer science skills. If you're going to merely analyze data, you can technically get away without it, but some companies (e.g., Facebook) expect all technical employees to know computer science.

Computer science is useful even to pure analysts because it will help you implement algorithms not covered by your libraries.

I make use of recursion and dynamic programming, for example, but I wrote production code. That's my deliverable, after analysis and prototyping.