Mantle was developed based on the Xbox One's API's, not the other way around. Anand did a full article about it and an update in the 280X review. Maybe read it:
http://www.anandtech.com/show/7400/the-radeon-r9-280x-review-feat-asus-xfx/2
Ryan Smith's unsubstantiated opinion is now a fact? Some of the stuff he said isn't even accurate. I addressed that article in the mantle thread and predicted AMD's API wouldn't be on either of the next gen consoles and that it has nothing to do with the API's on the consoles.
MS develops their own graphics API, D3D, Mantle was not a base, it was never on the XB1, AMD never designed the API used for it. And yes, the XB1 has DX, no it isn't the same as what we have on the PC, yes the feature sets are almost identical. It's completely irrelevant that they aren't the same, the collection of API's available on the XB1 are still called DX. No one has said it's exactly the same as what's on a PC, doesn't matter that the functions you're calling behave differently, it's still DX cuz that's what MS named it.
All of this "low level" marketing hype being thrown around is relative. No one is writing machine code for 90% of their engine. People act like there's some magic involved, you aren't writing nothing but asm. The existence of the API means you aren't writing "to the metal", the API is managing stuff for you, it's doing the communication with the hardware, albeit efficiently on the consoles. Both the PS4 and XB1 have drivers for their graphics API's, no coding to the metal. The real difference is primarily that on a PC you have all kinds of crap, user mode and kernel mode drivers, costly changes for the CPU to switch, a massive HAL, obese drivers, D3D doesn't permit you do everything you'd like to, etc. The consoles do offer lower level access, but inefficiency and whatnot along with D3D maintaining strict compatibility is the limiting factor on the PC.
Mantle just like the proprietery API's is able to bypass the abstraction layer and do work directly in the GPU.
It's no crime to conclude that AMD will have a HAL; because they're aren't to keep selling the same GCN 1.0 GPU's for the next 100 years. When they release new GPU's, they need the HAL to preserve compatibility. That's why it's there in the first place, so that you can write and forget. If AMD changes their architecture without a HAL there's going to be problems.