Which "millions of lines of code" are compatible with POSIX without having any Linux or macOS specific code? Or being compatible with Windows for that matter, which is POSIX compliant just in the name?
I think you're hugely overstating the importance of POSIX, not to mention downplaying the fact that POSIX really isn't sufficient requirement to not have to do any code porting.
I think you're hugely overstating the importance of POSIX, not to mention downplaying the fact that POSIX really isn't sufficient requirement to not have to do any code porting.