The seamless part is new, very new once a GPU is initiated it's locked, the fact that they can turn it on and off without a full reboot is quite amazing.
If you run a hypervisor setup with multiple GPU's you need to make sure that the UEFI and the hypervisor/main OS do not initialize them until they are passed through to the guest VM and after that you can't recycle them easily and usually need to reboot the host if you want to pass them through to another guest.
Is that any different from how Windows has restarted the GPU and reloaded the driver whenever a the video driver crashes since Windows 7? I think the new part might just be userland software being able to take advantage.
Even that's not new. I had a circa-2010 HP Envy laptop would switch from discrete to integrated graphics when you unplugged, and had a tray icon to manually switch between them without rebooting.
If you run a hypervisor setup with multiple GPU's you need to make sure that the UEFI and the hypervisor/main OS do not initialize them until they are passed through to the guest VM and after that you can't recycle them easily and usually need to reboot the host if you want to pass them through to another guest.