I meant a runtime that has to be installed separately. It's possible to statically link a C runtime if you use musl, for example
A language runtime remains one, independently on how it was linked into the binary.
A language runtime are the set of operations that support the language semantics, which in C's case are everything that happens before main(), threading support (since C11), floating point emulation (if needed), execution hooks for running code before and after main(), delayed linking, and possibly more, depending on the compiler specific extensions.
I can give other non-GNU/Linux examples.