They never said that they had to use c because go is not a systems language, so your assertion looks wrong. They wanted to avoid using unsafe.
In c everything is unsafe by the way, so it makes it less of a systems language?
No Go isn't a systems language because for one you don't have direct control over memory if you need it. For instance Go doesn't even have the volatile keyword which is essential in many cases when interfacing with hardware. The paper you linked laments this as well.