Long before Rust or Go existed, “systems languages” were commonly the ones you can write a whole system in to run on hardware, like C, Pascal or C++. I’m not opposed to that definition changing, but it certainly hadn’t when Go came out.

I agree that Rust enums should have been called unions, though.

While that does not match my memory, it works too. An assembly language is part of Go, so it fits among the languages you mention. The only constraint it imposes is your imagination (and what the hardware is capable of). However, it remains that "systems language" was caveated as being for network servers specifically. But no matter how you slice it, I think we can agree that Go isn't a scripting language, so it must be a systems language.

Rust does use enums under the hood in order to implement sum types, so the name as it is used within the language is perfectly valid. It's just not clear how that turned into nonsense like Go not having enums (which it does).