The difference between coreutils' and systemd's components is that changing the implementation of one coreutils program (let's say "ls") will not require you to

The difference between coreutils' and systemd's components is that changing the implementation of one coreutils program (let's say "ls") will not require you to modify others (like "cat"). Not only are they separate modules, but also they are loosely-coupled by design. Contrast this with systemd's modules, which are much more tightly coupled.