OS X has a lot of FreeBSD code, but it isn't a OS running on top of FreeBSD.

Lowest level of user-land perhaps. Plus some sections of the kernel were adapted from FreeBSD, IIRC.

It's close enough for tech-blog work, I suppose.

OS X (and derivative iOS) are essentially proprietary GUIs put on top of a FreeBSD base system on top of a hybrid Mach/XNU kernel with a bit of FreeBSD mixed in.