You believe that you’re more intelligent than they are because you are able to do one task better than them (coding), yet it sounds like they were able to do another task better than you (being able to successful convince you that they were more intelligent). I’m not sure why the latter should be ruled out as a sign of intelligence.
Actually, what I'd been getting at was that if they could convince me they were a better programmer than they really were, I could probably also be convinced that someone was more intelligent than they really were by similar means. If someone did convince me they were more intelligent than they really were, I'd have a harder time finding out I'd been fooled, for the same reason you mentioned.
I wouldn't take fooling me as a sign of all that much intelligence, though. I don't check the things people say about themselves for reasonableness very carefully. (Either not enough mental RAM, or force of habit from not having had enough RAM in the past - social interactions take more mental effort for me than for the typical person.)
I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.