It's not really surprising that people's intelligence seems to be rarely overestimated, though, is it? Smartness is impossible to fake, but you can fake stupidity.

In a hypothetical world where social skills (presentation) and IQ are inversely correlated, where you don't know they are inversely correlated, and where you spend most of your time interacting with people in the 140+ IQ range, a person with a 120 IQ could come along and impress you to such a great extent that you immediately hypothesize that their IQ is in the 160+ range, until you see something of substance.

If intelligence is underestimated in some, and talking to someone leads you to believe you can estimate their IQ, and people with better social skills initially appear more intelligent, then it is possible to overestimate intelligence. You just start with the benchmark presentation of someone you know to be intelligent, and when someone who is less intelligent but presents better than them comes along, you overestimate their IQ.

Happens to me all the time... (practicing overestimation, not being overestimated). One of the biases I've struggled with most.