I think these things are true for some people, but I think there's also a sampling bias when it comes to generalizing these things for the broader population of smart people.

IMHO even just the phrase "smartest person in the room" already frames the conversation in terms of stack ranking and some form of implicit value judgement on being smart, and this in turn opens to the door to all kinds of squishy emotional bias and self-identity issues that go well beyond any objective measure of smartness. In short, I think someone who even thinks in terms of "being the smartest person in the room" is likely to have their ego tied up in the self-perception of being smart, and thus more likely to feel the lack of intellectual stimulation you describe. Whereas I hypothesize there are many equally smart people who do NOT have their ego tied up in being smart, don't see the world this way, and simply don't show up in the data for smart people and their relative level of unhappiness.