The only mistake the OP made was in acting like they hadn’t seen the question before, but other than that everything was fine. These code screen questions are usually a basic gut check and the real evaluation comes later at the on-site. Honestly the interviewer did a bad job though because the question clearly gives an advantage to someone who has seen it before.

Once I had a candidate struggle with my screening question, flattening an array of nested arrays, and that’s fine because I normally use that as an opportunity to see how they ask questions and work together on finding a solution. But the candidate went silent for about 5 minutes and then came back with a perfect solution, using iterators, which are a pretty obscure JavaScript feature. I later found that exact code block on the MDN site, and I was pretty shocked at how brazen an action it was, because if they had just asked for direction and admitted they were struggling they could have still passed the interview.