Caveat: we may be using different definitions of "system programming". The one I'm using is code that is fairly close to the system, i.e. will call into libc or into the kernel/libSystem/etc. as well as calling more-or-less directly into a bunch of system-specific .so/.dylib/.dll.

In my experience as a system developer, you need to invest some time into understanding the invariants expected/promised by the libraries you're calling, but many of them map nicely to static types. Of course, if you're implementing e.g. a IPC or RPC layer, you need to deserialize (and validate along the way) your inputs, but there are very few systems that do not need to do that regardless.

Code that uses libc is just normal code.

I don't disagree :)