and you can add powerful external GPU boxes that connect via Thunderbolt
Unfortunately I'm afraid that this is not available yet, at least in a form of a proper functioning solution that is officially supported, perhaps in the future...
So it is upgradable just externally as Apple said
or it's upgradeable, but not in the "traditional" way we have been all following, till its launch in 2013 etc.
Anyway, imho, in present time the upgradeability of the GPU(s) is still a large question mark.