Sivers has it right, and I'm disappointed by the reaction here. People are smart. This should be your baseline. From there, people may do or say stupid things. It may become pathological. But the person has not lost his capacity to be smart (barring some physiological damage). It may have atrophied, like an unused muscle, but the capacity is still there. But even this is getting ahead of yourself--how would you know…
I completely disagree with the conclusion. If I go along with the premise that people are merely "acting" stupid, that is all the more reason to casually dismiss them. I can have patience with people that have limited capabilities, but people who choose to act stupid? Being semantically correct isn't particularly relevant. It's how you deal with consistent stupidity (and I'm not talking about the occasional brainfart…
You assume it's a choice, as if at the brink of decision, someone says, "I'll do the stupid thing." But in real life, you see people motivated by ambition, pride, fear and desire. And if that drives you do stupid things, to argue for stupid positions, does that make you stupid? Well, if limit ourselves to "stupid" always being contextual, if we understand "is stupid" as "acts stupid in a given context", then sure.
Likewise, if someone doesn't care about the things you care about, and consequently doesn't focus their attention on them to the same degree you might, does that make them stupid? Or just uninterested?
It's not a semantic issue. You are dismissing people outright, you are creating an alternate universe in your mind that through hubris drifts further and further from reality. Invariably, there's something else going on, something more interesting and more true to reality than just "he's stupid", and you miss that. I'm not arguing that you necessarily should engage with such people, but neither should you box them off with pat pronouncements on intellectual capacity.