You cannot fork systemd in practice; it's enormous, and its components are tightly coupled with complex, non-stable interfaces between them. So while you have access to the source code, you do not have the practical ability to exercise the FSF's four freedoms.
GNU/Linux was created as a rewrite of Unix not because Unix was the best operating system around, but because it was a design that could be replaced, changed, and improved piecemeal. GNU were able to write improved open-source versions of the components of a Unix system - such as init - piece by piece, and test them out and use them on existing Unix systems, rather than having to rewrite everything before they could do anything. If those older Unix systems had been designed like Systemd, that would not have been possible, and Linux would never have got off the ground.