Autistic spectrum disorders then manifest in systemizing behaviour, but that doesn't mean only people with autism are systematic.
Another phrase you could add to your vocabulary to describe that is "broad autism phenotype" (BAP). People with BAP have significantly more autistic traits than the average person does, but not to the extent that a diagnosis of ASD is warranted.
BAP is very common in two groups of people (i) close blood relatives of people with ASD (parents, unaffected siblings, etc); (ii) people in STEM professions. (Of course, those two groups are overlapping–ASD is more common in children of STEM professionals than in the general population)
The term "systematising" is specifically associated with Simon Baron-Cohen's theories of the nature and causes of ASD, which are controversial. By contrast, the term "BAP" is much more neutral, it doesn't presume any particular theory about what ASD is or where it comes from.
Personally I don't find the Empathising–systemising theory convincing, it seems as a false dichotomy to me. On the other hand I just remembered that my father's use of the word systematic in the above sense predates this theory because that's how he happened to describe me when I was a child.
ADHD and ASD have a lot of overlap. A significant minority of individuals with ADHD (20-30%) have a substantial degree of autistic traits. Many of that 20-30% would have enough for a comorbid ASD diagnosis, but not all of them. If you zero in on the individuals with ADHD who have substantial autistic traits but not quite enough to justify the comorbid diagnosis of ASD – do you say those individuals have ADHD with BAP? Or do you say that they just have ADHD, and that the varied symptoms of ADHD can sometimes include autistic traits?