The claim that systemd is modular is a brilliant marketing gag that rests entirely on how you define modular: software parts that are separated by clearly delineated interfaces can be called modules without being exchangeable. So in the eyes of the systemd guys the thing is perfectly modular, but as the users noticed, no part is exchangeable, which is what the other camp wants. And both camps get hung up over the contradictions without realizing that they need to redefine their nomenclature first.