The user takes the system with them when they put their phone away. Sure a designer usually uses a computer to shape the structure of a system. But the desired effect is psychological, manifesting in conversations and decisions. And the conversations and thoughts of the user are not just effect, they are also cause, changing the new system itself; a good app doesn't just suggest actions, it anticipates what you'll want to do with it next. These secondary features are designed by the user and discovered by the developer, not vice versa. This observation fits really well with lean principles (2, 3, and 6 specifically), but that's a digression.
The system I'm working on now, I want to not only be capable of going offline, I want it to be capable of going offdigital. What I'm writing will be like supercharged training wheels; one may never want to get rid of it but one can.
So HSI is HCI with an emphasis on systems. Both linked Wikipedia articles hint at the generality of the field beyond the digital, but not sufficiently. Since computation dominates these fields so completely, perhaps designers with an offdigital goal may find a new acronym useful. Or maybe the world doesn't need more three letter acronyms.