Yeah, bottom line, if you don't know an answer, then "I don't know" is always the right answer and the (ironically) most impressive answer. Plus you can always do things to mitigate, like talk about related or adjacent concepts a bit, state how you're willing to learn, maybe even take a guess at it. But if you bullshit you're either going to get caught, or, almost worse, get the job and feel like a fraud for months.

Always be hired or not hired for who you actually are!