… but that’s exactly how conversion to string works, so handrolling an algorithm would likely be around the same speed.

I don’t know enough about the question, but it’s possible that the interviewer wanted to test this specifically. I’ve had interview questions which were literally “implement itoa.”

In this case where itoa was part of a larger question, if I was conducting the interview I would let you convert to string for now so you could continue, but I would let you know that I wanted to revisit the digit conversion later.