DX12 and Async means squat if your able to code drivers and game to fully utilize all the stream/shader processors in the gpu not leaving sections idle. AMD never fully optimized their driver base for DX11"too much work on their part" vs Nvidia. This is why they(AMD) created Mantle and pasted it onto Vulkan and DX12 followed the same suit making the Devs responsible to code the hardware usage to able to have lower level coding ability. Almost all DX12 titles are DX11 based with DX12 wrapper which is why Nvidia tends not to see any real gains or even performs worse than plain DX11. For this game though looks like AMD kept up while Nvidia dropped the ball.
Dice, being the studio tasked with providing EA with a multi-purpose in-house engine for all it's many studios to use has the technical know-how to create full native DX12 support and then back-port it to DX11.
BF4 is one of the few games to have native Mantle support at the time when Mantle/Vulkan had very little documentation, this shows that this will not be just a DX11 wrapper but something built upon their previous low-level experience/codebase.