I've always understood this claim as a particular (rather restricted) interpretation of "systems" code - meaning network servers, basically (maybe also CLI applications?)
I'd always thought of "systems" as including things like writing standard libraries, kernels and embedded code. But I guess there is a decent slice of other systemsy stuff that Go is good for.
Whereas something like Zig feels like it could really be a better systems programming language in general that C, in all of its different application areas.