Well, they used the phrase "systems language" when they introduced it, and it still appears prominently in the FAQ, but what they had in mind vis-a-vis what-exactly-constitutes-a-system was very different. (They were thinking more along the lines of a cloud infrastructure language.)
If you watch this excellent panel discussion from this year, you can hear Rob Pike express how they later regretted how their poor choice of terminology created such confusion:
