A person with borderline intellectual functioning can absolutely have capacity to understand a contract sufficiently to agree with it when its terms are explained to them in clear plain English, yet lack the same capacity when they are presented in dense legalese. Legal doctrines of "capacity" tend not to deal with that situation very well, because they focus on the capabilities of one of the parties rather than the form in which the contract is presented. Also, a lot of people with mild cognitive issues (not just borderline IQ, also other issues like age-related cognitive decline, early stage dementia, early stage hepatic encephalopathy, etc) are unaware of those issues, in denial about them, or too ashamed to admit them, so may not benefit from legal rules designed to apply to them specifically, whereas they can stand to benefit from legal rules (like demanding unexpected clauses to be stated prominently to be enforceable) designed to apply to everybody.