That is completely untrue. While the BSD syscalls are supported, Mach syscalls and Mach IPC are also heavily used, especially internally at Apple.

Just one simple example: run "sample <process>" (where <process> is any Cocoa application) on OS X. Pretty much every thread will have an event loop powered by CFRunLoop, and they'll all be stuck in mach_msg_trap waiting for an event to occur.

Also, pretty much any sort of semi-complicated IPC on OS X is done over Mach IPC (e.g. passing a large bitmap copy-on-write from one process to another). Take a look at the ipc directory in the Chromium source, for example.