I largely agree with this, although just POSIX by itself isn’t usually enough to write first-class software.
I’m currently working on a event loop library (https://github.com/davmac314/dasynq) and through use of testing macros (including for POSIX features generally, but also for specific OSes) I have managed to get it working on Linux, OpenBSD, FreeBSD and MacOS without using any autoconf or similar script - it’s pretty much just tweak a few makefile variables and then run make (or gmake). However, it needs either kqueue or epoll; POSIX just doesn’t have any high-performance equivalent to these. I might get around to doing a poll()-based backend at some stage, but it’s got to be considered a fallback option.
But Windows is pretty posix compatible? Unlike linux, they even have the sticker!