Well technically it just is borrowing the apt packaging system from debian, it is not debian. Underneath it still is darwin, and use the XNU kernel (vs the linux kernel)
The problem comes when people start mixing Linux and GNU. Linux is only the kernel, it really isn't useful as a standalone. GNU is the free software that runs on top of *nux like systems. GNU and Linux have become very related but they still are separate. And since Darwin (iOS/OS X) is unix like, using the XNU kernel (technically a merge of the BSD kernel and Mach kernel), GNU software runs on it.
Hope I don't come across as snobbish, but this is a subtly that many miss.