Intelligence, in the real world, has a variety of meanings, from the more specific IQ to Howard Gardner's multiple intelligences of which IQ is just a small portion (probably a combination of verbal-linguistic and logical-mathematical, leaving the other nine mostly untouched).
I've also known many people who are quite intelligent in terms of the depth of their thought, but either have a difficult time translating their thinking to words/actions, or slower in their processing. And I've known people who are extremely intelligent in other forms of intelligence--say bodily-kinesthetic or spatial-visual, etc--but not as much in terms of verbal or logical. I mean, how many of us know someone (or are someone) who "sucks at math" but is highly intelligent in other ways?
But let us not forget that D&D's Ability scores are just that: abilities. I used to think they were wrongly named, and should be called "Attributes," but if we instead take the word literally--Abilities--then we can see them as a character's capacity to enact actions through various means or aspects of themselves.
So INT doesn't have to be the depth of a character's thinking, but their ability to process certain types of information, mostly related to logic and conceptual thinking. WIS, in a way, covers some of the other forms of Gardner's intelligences: existential, spatial-visual, intrapersonal, maybe naturalistic. And CHA touches about interpersonal (social-emotional) and perhaps musical intelligences, and of course DEX covers bodily-kinesthetic.
So a low INT character need not be an idiot, they're just bad at logical processing, and especially if they're higher in WIS or CHA, is "intelligent" in different ways.
EDIT: I would also add that a high INT character could be an idiot - perhaps emotionally clueless or lacking imagination or intuition. I think the mistake is in thinking of INT as all aspects of intelligence, rather than a more narrow band of rational-logical thinking and processing.
