A developer who is self-aware and mature enough to answer "I don't know" is good to have around, and usually more valuable to a company than someone who freaks

A developer who is self-aware and mature enough to answer "I don't know" is good to have around, and usually more valuable to a company than someone who freaks out or starts making stuff up once they're outside their current range of knowledge. It's possible the interviewer was screening you for that as much as for your technical skills.