You might want to look up the definition of that word. The design is definitely not proprietary. Just because no one has chosen to make the video cards in no way makes it proprietary. There is the whole firmware issue but even then that still does not make the cards proprietary.
It's their practice, their policy.
If you check the Gpu's form factor, the special design to fit in, the way they are cooled down, the special connectors,
the fact that the one of them has the ssd's pcie connector built in (and
this is not the industry standard connector), the firmware prototyping method, the drivers, the special way they connect with displays and they compute, etc.
It is clear that Apple did it's best for one more time to lock down the internals as much as they could, for their reasons (technical and economical), as they prefer to buy a whole new device from them than upgrade your current one.
You can see this in every current device they sell iOS or OS X based, everything are soldered in, batteries, ram, gpus, screens (glued) and pretty difficult for a user to reach them, they have also made custom screws to discourage people from opening their devices.
So imho the gpus have not industry standard design, they are locked with several methods to discourage third party manufacturers to built something compatible, the gpus firmware is outside of the cards, they 're completely custom made and locked -> proprietary.
Have a nice day.
