It's not the same version of OS X that ran on Apple's computers. The "it's OS X" was more for marketing, they just share the same "core".

You could argue that the iPhone currently still runs macOS if you used the same definition today. They share kernels (iirc Apple always kept the ARM patches to Darwin closed-source), BSD-based userlands and the iPhone used versions of the macs application libraries.

A big difference is the iOS and macOS use different compositors.