The root comment is saying the interviewee knows the language well enough to consider one solution better than another solution. And this was misinterpreted by

The root comment is saying the interviewee knows the language well enough to consider one solution better than another solution. And this was misinterpreted by the interviewer as not knowing the language. An effective interviewer would have asked "Why are you doing it this way?" instead of assuming - wrongly - it was all the candidate knew. This actually matters. Before you even get to coding skill you want people who can parse reality accurately, and not make incorrect assumptions about what's happening in front of them - either out of narcissism and arrogance, or because of poor communication skills, or because they're following a set process which is bureaucratic and inflexible and operates with a poor signal to noise ratio. (Among other possible reasons.)