People usually aren't trying to protect their drive contents with encryption while the drive is mounted and the computer is running. If the drive is encrypted, anyone trying to gain access won't care if OSX has a backdoor because it will all be encrypted in the volume. The only thing that will matter is a backdoor allowing decryption of the volume.

You could use your argument to state that someone paranoid enough to use encryption just shouldn't use a computer at all.

My understanding is that it's not considered "true" open source by many people, because of the license that it's released under, but their website gives easy access to all the source code.