Well, I wasn't one of those arguing that systemd should be portable across OSes: init systems generally aren't, not even the simple ones. I was arguing against your claim that this is universal in other sorts of software: it is not. Lots of software supports portability in various ways, be that compilation with different compilers (binutils, GCC), or running on different platforms (glibc, to an extent)... and huge amounts of software doesn't care e.g. what glibc or binutils or kernel version it was built with. The coupling is not as tight as you suggest.