DX 12 apparently allows iGPUs to work in tandem with any kind of dGPU, so Intel is suggesting that only its desktop CPUs can do that, since AMD's desktop processors do not include iGPUs. However, AMD still has APU models, though not as powerful as the desktop chips, but they can easily take advantage of this feature, as well.
https://www.notebookcheck.net/Intel-CPUs-can-boost-Nvidia-AMD-GPU-performance-thanks-to-unused-iGPU-power-and-DirectX-12-potential.459102.0.html