It all hinges on the definition of systemic. This is an interesting article:
https://www.medicinalgenomics.com/powdery-mildew-systemic/
Definitely not peer reviewed, primary research, science but interesting none the less.
I grow in the Mojave Desert. I don't run air conditioning and my grow is indoor hydro. During the summers my humidity never tops 10% so why did I have PM showing up in that Bodhi clone each generation and worsening? I flowered it sitting right next to many other strains who were never touched by PM.
Although it is easily debatable it is not technically a systemic illness it was only treatable by a systemic. All the environmental and topical therapies failed. This leads me back to my experience in medicine with topical fungal infections, for example distal subungal onchomycoses is commonly caused by a dermatophyte. It is still a topical illness however it requires a different form of treatment (systemic), than simple ringworm or athlete's foot once it has burrowed under keratinized protein. Simple topicals were not effective.
So the taxonomy isn't as important to me as the functional treatment and that experience has been borne out on these grow forums. The semantics vary from the classroom definitions. I gave up and now pretty much go with the colloquialisms, hope that helps.