I wish the author had spent time addressing that theory specifically.
Most neurotypical people seem to experience jittery effects and being extremely alert while for people with ADHD it can actually make them sleepy.
Of course everyone's brain is different, it is just a correlation with many exceptions. So yeah, caffeine works differently on different people.
Personally, as someone with ADHD, I have a crazy caffeine tolerance. It helps me somewhat with focus but I don't get jittery.
The narrative that 'stimulants calm down people with ADHD but make neurotypicals wired' never sat well with me. But I totally believe that it makes you and many other people calmer without any friction.
There must be a wide range of physiological causes and behavioral circumstances that lead to an ADHD diagnosis, such that people like me take a baby dose of methylphenidate or else I get paradoxically overstimulated and distractible and physically uncomfortable.
Or maybe my stimulant tolerance is unrelated to the ADHD and it's just enzymes. I think I had a flag for one of those SNPs that makes me sensitive to caffeine...
There are people with ADHD for whom meds don't work at all, some for who it works in very low doses, some that need very high doses, everyone is different. Same with reactions to coffein.
Diversity is the norm in nature when talking about individuals. ADHD is super complex and how stimulants work is also super complex and the interactions, well we barely have any idea. It doesn't really say anything about your ADHD that you are more or less sensitive to stimulants other than that your are more and less sensitive to stimulants.
Like you wrote, it could be just some enzymes or whatever. Humans are just crazy complex. It is still useful to talk about fact that a statistically significant subset of people with ADHD react differently to caffeine than most neurotypicals.
For what it is worth, I envy you a lot being more sensitives. I can kill five, six, seven cups of coffee and not feel anything. It sucks.