I'm pretty sure that is a myth, and it only appears that way because the circumstances are different:
People with ADHD are treated on a very low dose, and slowly titrated up which makes you much less likely to feel high. People without ADHD that are illegally using stimulants aren't getting treated by a doctor, and aren't going to slowly titrate up the dose, or take it consistently in low doses over a long period of time. People with ADHD sometimes do feel a small high or euphoria for a short time when they first start a new medication.
People without ADHD but using adderall illegally, e.g. as a "study drug" are not getting a high or euphoria if they consistently use a low dose like an ADHD person.
I have clinically diagnosed ADHD, but even the lowest possible dose of adderall makes me extremely high in an uncomfortable way and unable to sleep for days. Most likely, I also have a liver enzyme mutation that causes me to not metabolize amphetamines properly. Ritalin I metabolize quickly, and don't feel a high at all.
You can't diagnose ADHD by giving a medication and seeing how people respond. Even people with ADHD respond very differently to the same medications.
There is also the fact that medication with ADHD can give people executive control over hyperactive physical movement, so they may seem to be "slowing down," which kind-of looks like the opposite of a stimulant effect, especially to adults watching hyperactive kids calm down. But that is basically the opposite of what is actually happening- the increased stimulation allows the brain to regain executive control of behavior.
I'm normally on a very minimal dose. I have, prior to getting timer tops and forgetting that I'd already taken med, taken >100mg of dextroamphetamine without feeling high and without titrating up. Instead, I got very focused and methodical to an uncomfortable degree, but there's absolutely zero high or euphoria.
Across ~4 doctors (1 PCP, 3 pysch), none have titrated up. They've ballparked and said things like "let me know and we'll reduce if you can't sleep and increase if it doesn't work. If you want, try doubling up or cut it in half (for non-XR)".
Given the variation in dosage visible in the literature (such as this case report of megadosing: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3407707/ ) I'd suggest that there are multiple underlying physical causes for the condition described as ADHD to the extent that sweeping statements like that aren't accurate. E.g. my (very different) experience and your experience being different indicates that we probably have different underlying causes, not that one is a myth or misperception and the other is the real take.
Although I agree that is possible, the extreme variations in dosage between individuals could be explained simply by variations in the liver enzymes that metabolize stimulants, and don't necessarily point to different underlying causes of ADHD itself. However- I think the fact that entirely different classes of drugs, including non-stimulant medications, seem to work better for different people with ADHD does point to what you are saying.
In my case I can't tolerate even 2.5mg of adderall, and I also have hypersensitivity to several other non-psychiatric medications that happen to be metabolized by the same enzymes as adderall/amphetamine, so I am nearly certain it is a liver enzyme issue. I do tolerate a reasonably high dose of methylphenidate (ritalin) without any euphoria or insomnia.
People with ADHD are treated on a very low dose, and slowly titrated up which makes you much less likely to feel high. People without ADHD that are illegally using stimulants aren't getting treated by a doctor, and aren't going to slowly titrate up the dose, or take it consistently in low doses over a long period of time. People with ADHD sometimes do feel a small high or euphoria for a short time when they first start a new medication.
People without ADHD but using adderall illegally, e.g. as a "study drug" are not getting a high or euphoria if they consistently use a low dose like an ADHD person.
I have clinically diagnosed ADHD, but even the lowest possible dose of adderall makes me extremely high in an uncomfortable way and unable to sleep for days. Most likely, I also have a liver enzyme mutation that causes me to not metabolize amphetamines properly. Ritalin I metabolize quickly, and don't feel a high at all.
You can't diagnose ADHD by giving a medication and seeing how people respond. Even people with ADHD respond very differently to the same medications.
There is also the fact that medication with ADHD can give people executive control over hyperactive physical movement, so they may seem to be "slowing down," which kind-of looks like the opposite of a stimulant effect, especially to adults watching hyperactive kids calm down. But that is basically the opposite of what is actually happening- the increased stimulation allows the brain to regain executive control of behavior.
Here is a journal article reviewing the evidence and debunking this idea: https://www.nature.com/articles/1301164