I would agree that further consideration of twice exceptionality may be in order. Depending on where you are, qualifying him for an IEP may include gifted IEPs. Even if not, eligibility for an IEP in the face of A/B grades is actually a likely sign that the school is particularly responsive to the needs of twice exceptional students, as many districts would stop at the grades, and not look further for any unique learning qualities. IEPs are not necessarily about low grades, as the eligible disabilities include, for example, emotional (e.g., significant anxiety or depression), autism (which can quite easily co-exist with strong academic skills) and other health impairment (which is typically where ADHD resides). In each of those cases, the skills in need of remediation in order for the student to access their education are not essentially academic in nature. E.g., emotional IEPs might address emotional regulation or coping skills, autism IEPs might focus on social skills, and health (ADHD) IEPs typically are written to work on executive functions and study skills. Even if a student is posting strong grades, if it is simultaneously (for instance) causing them emotional distress or requiring excessive time or effort, an IEP team can legitimately find that there is a need for specialized skills remediation.

That they proposed an IEP strongly suggests that the school is seeing behavior in school that indicates some kind of lagging skill to them, even if it is not academic.

(I would echo the comment above that an OT is not qualified to diagnose/rule-out learning disabilities. Be very careful about providers that make pronouncements outside of their scope of practice. They may or may not know what they are talking about in terms of fine-motor and upper body skills, but speaking out of school makes me a little leery even of their OT assessments.)

On the ADHD-relevant measures: there are many dimensions of attention and working memory; it is unclear which aspects were assessed. In addition, ADHD comes in varieties that are notable for one or both of two aspects: attention/working memory and impulsivity. Most individuals with ADHD have the primarily inattentive type, with the second largest group combined (both inattentive and impulsive), and a smaller group in the primarily hyperactive (impulsive) type. (Some research finds a different distribution weighted toward the combined type in younger children.) A thorough assessment for ADHD should examine multiple dimensions of attention and inhibition skills.

If the school conducted the evaluation, then you have a right to written reports. If the reports do not include details, you can request an additional meeting with the evaluator to review those results. They may or may not want to give you a copy of details, for intellectual property reasons related to the publishers, but even in that case, you can request (with a signed release of information) that they share all of the test details with a qualified evaluator of your choice, which would give you additional options for interpretation of the testing by someone with deeper experience with twice exceptionality and giftedness.

If you had this evaluation done privately (not by the school), then you will have to go through this process with your private evaluator. Unfortunately, I have seen plenty of private evals that are unsatisfyingly short on specifics, so no guarantee that a neuropsych will be any better than what you have now. (Actually, after a couple of decades reading and writing evals, the worst neuro/psychological evaluation reports I have ever seen were from private clinic-based evaluators, not from schools. Not that there were a lot in this category, but those were the ones with no scores at all.) But at any rate, you are the client for the private evaluator, so one hopes they will be more communicative. Moving forward, do some rigorous research on prospective evaluators, including references and interviews, before you sign on with anyone. And make sure you provide future evaluators with all prior testing records, so they are not blindsided by invalid results from retesting too close in time.