I know this is a joke, but I'd be careful about assuming: I had a trained therapist diagnose me with ADHD ... incorrectly.
It turns out, there are other conditions out there that can appear to be be ADHD. If a trained therapist can confuse them, outside observers certainly can too, so it is possible that "very ADHD-acting" person you know might not have it.
(Or, especially for females, the reverse can be true, as not everyone with ADHD manifests with the same trademark hyperactivity.)
I understand this is your personal medical info, but could you drop any hints as to the nature of the misdiagnosis? I've learned late in life that I am neurospicy and learning from other's experiences in this domain has been quite helpful.
I listen to a lot of psychiatry podcasts, and they generally think that ADHD is being seriously overdiagnosed at this point.
Executive dysfunction, especially that which was not impairing as a child, has a lot of potential causes. Anxiety, depression (yes, ADHD can cause these, but these can also cause executive dysfunction in people without ADHD), OCD, Bipolar (manic people in particular often attribute worsening of hyperactivity and distractability to ADHD instead of mania), Schizophrenia pretty much universally is related to cognitive symptoms and lowered motivation even when not psychotic, and borderline personality disorder can cause changes in interests and identity.
For many of these conditions, stimulants can act as a band aid on something else. It can superficially make people feel better (of course, it's speed), but it can distract from other symptoms that are impairing functioning, and prevent people from seeking long term solutions.
I can roll with that. I do have executive dysfunction and other "fun" facets of a funky neurobiology. I also recognize that every body and mind is different and it's a personal journey to find the right tools.
I recognize that non-medical options should always be front line approaches but there can be a chicken and egg effect as to making that happen.
In my youth I adhered to a "better living through chemistry (wink wink)", I do now too (rip Mitch Hedburg) but the wink wink part is very rare and disciplined and now it's all about pharma and supplements to wring what I can out of what I've got left.
On the non-medical options front, I will note that my lifelong hyperactive and neurotic tendencies dramatically improve with a 3-4 day a week distance running training regimen. I enjoy running enough, even though it is very difficult, that I've kept up with it for several years. I'm not as consistent about this, but I also found yoga classes to be a way to make mindfulness fun and easier.
That's not to say I'm not on the medication train (despite moaning about medications a lot). I do personally avoid taking any controlled substances. This is easy because I hate how stimulants make me feel, but in general I worry about the effects of having a direct reward signal from taking a pill.
Exercise is one hell of a drug, to be honest. I suppose you shouldn't exercise for a few issues in particular, but in general, boy.
Differentials for ADHD include: panic disorder, metabolic issues, anxiety disorder, depression, adrenal disorders including deficiency and excess like pheochromocytoma, bipolar, autism, learning disorders, personality disorders like borderline and antisocial, OCD, trauma including PTSD and CPTSD, sleep disorders including narcolepsy, sleep apnea, restless leg syndrome, insomnia, ironically substance use disorder like cocaine and amphetamine addiction, thyroid issues again ironically including both hypo and hyperthyroid, seizures, hearing and vision issues, and life stressors.
So, uh, ... a whole textbook. And I probably forgot some.