Linux is mostly POSIX compliant. It has layers on top that are not POSIX, but that doesn't mean it is not POSIX or that you can't expect your POSIX apps to work on Linux.
Not at all, it means that many mistakenly use Linux syscalls as synonym for POSIX, thus making the same lock-in to GNU/Linux as Google, Apple and Microsoft do to their own systems.
*BSD devs jump of joy having to port GNU/Linux applications that depend on D-BUS, systemd, or any other Linux specific APIs.
Given that *BSDs also have many of their own extensions, (ie openbsd has many security extensions to the POSIX standard), that isn't the best example of a defense.
