Feel free to actually point at the inaccuracies and go into detail instead of general statements that say nothing.
...and didn't say calls using the Mantle API aren't compatible with Xbone (because they are since GCN is GCN, be it in the consoles or the PC, so if a call work on the Xbone it works on the PC).
Hard for me to word without touching "personal attack" stuff and still explaining where (i think you could be) you are wrong in believing that Mantle API is somehow enabling magic compatibility with GCN architecture in a cross platform way. (the API call works everywhere part of it)
GCN as architecture does not mix with words API call and "compability" in same sentence. Think about the following:
1) XBone will use DX11.2 derivative API, it might get some different features, extensions sure. But it is still DX11.2 API, more or less similar to what we have on PC, MS is not reinventing wheel. What they have optimized is what is below API. It is more like a shim, tailored for their APU, that happens to be be GCN. I doubt anyone is doubting that MS would use same DX11.2 even if they chose Intel APU, just features and shim would be different.
2) PC is also using DX11.2 API with a subset (or superset) of capabilities that XBone has. API is same, architecture is the same, but game engine has to operate on what is a massive stack of things DirectX API calls into DirectX Runtime, that interfaces with a driver that follows WDDM 1.x model (driver API if you like) that interfaces with hardware. Lots of overhead that is common for any architecture, AMD, Intel etc suffer from it.
So same API is what creates compability, you can be using DX11.2 on XBox one, with certain XBox like assumptions that are not currently valid on PC ( let's say you can count on hardware being able to do 100k draw calls per second or there is some magic to tile textures).
it is this
game engine compability between console way of game programming and PC way of game programming AMD is aiming to achieve. Using same render path and not having to do major logic changes in backend is holy grail for them! Again consider the following:
Xbox360 is using what is in spirit DX9 API with steroids, usage of this API is however different from PC. For example you can spam draw calls from all CPUs (pretty much have to do so, cause individual PCs are weak) and runtime will take care of them. Try that on PC and you will either fail outright cause AMD does not support this feature or have to write "special" path for Nvidia that supports it and you will probably still run in trouble cause of DX overhead. So major engine rendering backend rewrite is needed no matter where you start from PC or from Xbox360.
So this is simply not correct:
(because they are since GCN is GCN, be it in the consoles or the PC, so if a call work on the Xbone it works on the PC).
It is not correct because "how AMD will design Mantle API" is completely different question from "how will AMD implement Mantle on GCN architecture".
Neither Mantle nor DirectX cares about architecture below it as long as it supports required features, calls will work, get emulated/synthesized on new hardware with other hardware features in the future etc.