More likely is that you got biased from that conversation and that bias colors your later evaluations of them. A lot of people are biased like that, studies shows that, maybe you are one of the very few that would be the exception of these studies, but it is unlikely, much more likely you are among the large majority who overestimate how well they can read people in free form conversations.

perhaps you just overestimate your interviewing skill

I will say learning to interview is a skill like any other, I have no doubt the person you replied to was not born into the world understanding the nuances of technical questions and what heuristics to use to determine validity. Just like yourself.