How do you know they didn't just have heard about weighed median before and how to implement it?

Or did just more grinding on leetcode in general.

I am not convinced that what you call "strong" can be tested by something like that.

Interviews are just collecting circumstantial evidence anyways. I do not really think a single interview can conclusively prove that a candidate is strong. Within the limit of one interview, this candidate is strong enough. And I'd be happy to hear contrary opinions from the other interviewers.