Yes, but that's quite different from how language package managers are used. A typical C/C++ project doesn't tell you to build it via apt/rpm instead you are expected to provide the dependencies using whatever means you choose or they are included in the repo. This means you don't need to fight a package manager to get fully offline or reproducible builds for example.

RPM and APT packages are also usually not maintained by the upstream developer but by distro developers who care about making different packages work together so you don't get the dependency hell problem as a user.

Yes, C programs go to lengths to give the user choice. But it is recommended to use source packages for e.g. dpkg. And it's not too rare to have the .debian directory in the original VCS.