That is not correct. Mantle would work very well with Nvidia GPUs, if Nvidia decides to support it. As opposed to Nvidia’s Gameworks closed libraries, which would hurt AMD quite alot, if Epic decides to integrate those into the engine.
Most users will still be on Win7 when DirectX hits the market. Those will most likely not profit from DirectX 12, but they would from a Mantle API. DirectX 12 is also no option for the Linux and Steam OS users.
Achieving 90+ fps in 1440p+ in a graphically taxing environment, will, for most users, just be unachievable. Most people will not invest that much money in their PCs. So we will have to learn from the consoles and upscale the resolution to match the native resolution of the rift. You would still benefit from the higher clarity and absence of a screen door effect while maintaining high enough frames rates. That’s when Mantle could make a difference!