I wouldn't say there that far behind but its true DirectX hasn't made the significant changes it needed to make until now. But the truth is it was easier for Mantel because they(AMD) make the hardware but if they don't get Nvidia and Intel its worthless because those two companies control the vast majority of the GPU market. Yes they got EA and Crytek onbard but I guarentee you both companies are going optimize their engines for DirectX12. The fact they showed Forza on Nvidia hardware shows that about close to midway through development. By the end of the year they should have a beta DirectX12 available.
I think nvidia are going to add there support into mantle cus i can imagine DX12 being good for a wide range of hardware but for high end gaming a non bloated fast API that can be used to run on the latest cards is best. Whether DX12 mimicks mantle or not it will still be bloated so i think mantle will edge it out in performance and especially if they want 4K gaming to take off they ned to squeeze the performance out of the GPU's