This tweet was posted early 2014, so the x86 architecture had already entered the console space. I don't think anyone here is fit to challenge Carmack's words. The guy basically made more progress in 3D game engines than any other single human being.
No, Carmack is one of the most knowledgeable 3D pieline engineers out there. But that doesn't stop idiots from taking what he says out of context. I hope you're not one of those idiots.
He was specifically comparing the old DX9 API and it's CPU performance. He was basically saying: in certian scenarios you can squeeze out twice the performance of similar CPU + GPU PC hardware *running DX9/DX10*.
That quote, is, to anyone with half a brain and access to the internet, obviously not accurate to anything happening today or even years ago now.
DX11 significantly improved multi-threading when workign with the 3D API as well as overall CPU performance. It modernized the GPU API's enough that while there is still a much higher hit on the CPU on PC, specially when it comes to draw calls, the situaiton is just not anything like 2x on the GPU. Just look at that 750ti go! Dx11 means, so logn as you have somethign akin to an i3 or better, your GPU will perform about as well as if it was inside a PS4 (lower end CPu's might not have enough grunt for modern games however and 3D performance is likely to drop).
Enter DX12/Vulkan, and you are very close to parity in terms of possible optimizations on both platforms thanks to a thinner dirver and direct access to GPU resources (not to mention a much lower CPU hit on drawcalls). With these new API's the CPU hit is further reduced, making the quote officially outdated.
So the quote is pretty much irrelevant today, has been irrelevant for many years now, and yet I guarantee some idiot will still bring it up 5 years from now.