Imagine going back in time to the 90s and telling people that in 2016 Microsoft would bend over backwards to make sure Windows could run Linux ELF binaries natively in the kernel by converting syscalls. You'd probably be laughed at.
POSIX calls, which have always been there but require a recompiled binary, are different than the binary compatibility announced today. This additional step is actually quite an impressive feat of software engineering. To think that Microsoft would care so much about maintaining compatibility would have seemed ridiculous just a few years ago.