The creators of Go were on the team that built UNIX, and they clearly have a different meaning for the word systems.
They were using it to refer to infrastructure, without making a distinction whether it was distributed or local. Something like a compiler would be "systems" program under this definition.
I agree that distributed systems, and systems programming are generally accepted terms of the art in CS now, and even 20 years ago.
How modern is modern CS?
I took my CS degree around 20 years ago with focus on computer architecture, compiler design and distributed systems.
Distributed systems literature used in the degree went back to the early UNIX days.