I’m not sure what you mean when suggesting Linux is a singular implementation around which features are exclusively designed. There’s all kinds of software that runs on all kinds of different OSes. Userspace applications, for example, can take advantage of POSIX compatibility to ensure that they run on all platforms (Linux, BSDs, even Windows).
Does systemd have any similar sort of compatibility guarantee? Can I run systemd-whateverd on BSD? Can I run systemd itself on BSD? I’m pretty sure most other init systems support at least one other OS if not more. Would the maintainers even support merging patches that do this? What about musl?
Has that ever been true? I always assumed it was some sort of shadow marketing campaign to get people to look at reddit more. Pretending that one website is the only reliable source of answers on the internet is incredibly audacious, it always seemed very farfetched to suggest that