Why a database system should care about how processes get started is beyond me. Systemd is an entangled mess that every year subsumes more and more of the operating system, in a very non-cooperative way.
There are almost ten init systems. In every one of those init systems, one can run a process supervisor, such as runit or s6 or daemontools-encore, completely capable of starting the postgres server.
Every year, systemd further hinders interoperability, further erodes interchangeability of parts, and continues to address problems with WONTFIX. In the long run, you do your users no favor by including init-system specific code in Postgres or its makefiles. If systemd can't correctly start Postgres, I guarantee you that s6 or runit, running on top of systemd, can.
Postgres doesn't care which language makes a query to it. Why should Postgres care which init system started it? I hope you can free Postgres of init-specific code, and if for some reason you can't do that, at least don't recommend init-specific code.
Take a deep breath...
You are looking at a page about PostgreSQL with specifics surrounding installation on a machine running systemd. In that case it is naturally recommended to compile using the --with-systemd option to better integrate with systemd.
As the docs about that option say, "...This improves integration if the server binary is started under systemd but has no impact otherwise..." You are no more required to use systemd than you are to run PostgreSQL on Windows but the options are available to you.