What word would you use to differentiate design and non-design functionality?
I think the terms are usability Vs usefulness.
UX vs UI?
If one says "Good design is part of the functionality", one can also say "Interface is part of the experience".
"can I do something [at all]" vs "how [easy] can I do something"
vs “can I even determine whether or not I can do something”
Business logic
I think anyone who differentiates between design and functionality has a profound misconception of what design should mean, and are the kind of people who produce terrible interfaces because they just see design as prettifying the functionality. That's why you get buttons that don't look clickable, lines where they're not needed, no lines where they are etc. Good design is a holistic practice, and in the case of apps the interface should be an integral part of the functionality. There's lots of literature out there about things like affordances, but many people don't know about it or don't care enough to use it in practice.
I'm talking specifically about the app SyncThing, though. I use it and rely on it, and I haven't opened it in over a month. It works as a background service.