I never commit to memory anything that can easily be looked up in a book
The value of a college education is not the learning of many facts but the training of the mind to think. - Albert Einstein.
I wouldn't worry about giving correct answers, but how you approach giving an answer at all. I'd hazard a guess that they are aware of the relative impracticality of the question so they're looking out for something else: Approach.
On one memorable occasion, I've been in exactly the same situation. While interviewing for a Senior Systems Architect/Integrator position, I was asked a very technical question in a technology I had been very clear was not my strong suit at the start of our session. When the question came, I told them that while I couldn't give them specific pieces of the technology that would be ideal for the problem, I offered alternatives in other technologies that I was comfortable with. I even proffered a likely solution in the tech I was weak in, based on residual memory and how I felt it should likely work. The interviewer proceeded to offer his opinion on approach and we moved on
At the end of the session, my interviewer stated he was impressed with my performance, not because I had all the right API calls locked down pat; Because he could see that I was working out in my head, a path to the solution to the problem. He said something along the lines of not giving him crammed or canned answers and how he could observe that I was working on actually solving the problem rather than giving him something regurgitated from a textbook. I got the offer (and turned it down for another).
While your current response is fine, you should at the minimum approach the problem with the aim of providing viable alternatives to the absolute correct answer, based on experience, residual memory whatever you have. Make reasonable and educated guesses that shows that you're firing on all cylinders and you're a ferocious problem solver. Not trying at all might not hurt you, but it really wouldn't help either
You'll be fine.