WRONG. Mostly.
Performance comes from the processing elements in the GPU. There are only so many. NVidia is doing a much better job in DX11 so has not got much room to improve.
AMD DX11 drivers are worse, and is losing performance. With DX12 shifting the optimization towards the game engine then the drivers are not quite as important so the game can make better use of the GPU.
Even if AMD is now more focused on DX12 drivers, it's NOT fixing the lower DX11 drivers hence a performance disparity exists.
*There are other reasons, but it mostly centers around the fact that NVidia can't improve performance much when it's already using up most of the GPU's processing.
(People keep looking at this BACKWARDS. AMD isn't beating NVidia per say, they are just fixing an issue that they had themselves)
Other:
There ARE additional optimizations that can be made using DX12/Vulkan and both AMD and NVidia will do that and AMD may have the upper hand in terms of future performance relatively (how ASync Compute + low-level shaders work), but for now that's not the main reason.
Click-bait articles and fanboys have perpetuated this myth that NVidia just isn't ready for DX12. Not true.