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
This kind of setup has been allowed in the past by using AMD APU/GPUs and hybrid crossfire. However it has never brought any significant benefits and in some cases it even slowed down the framerate.
No, this has nothing to do with Crossfire or Dual Graphics. The dGPU still renders the game exclusively. The iGPU is only tasked for light stuff in AI calculations, and various post processing effects to relieve the dGPU and thus, give more performance.