It’s that it also decided to take over log management, event management, networking, DNS resolution, etc, etc.
If it were just an init system that would be perfectly portable. People were able to write software that way with sysv for years.
It’s that in order to do certain low level tasks on a systemd system, you need to integrate with systemd, not just “be started by it”. Now if a distro wants that piece of software, it needs to use systemd, and other pieces of software that want to be on that distro need to implement integration with systemd.
A dependency isn’t infectious, but a dependency you can’t easily swap out is, particularly if it’s positioned near the base of a dependency tree.
Almost all of my software can run on x86 or arm without any issues beyond changing compiler targets. It’s closer to how it’s tricky to port software between Mac and Linux, or Linux and BSD. Targeting one platform entails significant, potentially prohibitive, effort to support another, despite them all being ostensibly compatible unix like systems.