Basically, it means Optimus style and hardware-muxer style setups are obsolete, and display framebuffer and render framebuffer are entirely disconnected naturally; and also does not require any overt cross-architecture support (ex: Nvidia Optimus support in Nvidia's driver requires explicit Intel support; dma-buf solutions don't).
Presumably, OSX also can leverage this.
Also, in Windows 10, the window manager's compositor (equivalent thereof) does all the event signaling fine; ie, you could easily have a situation where a game is rendered on one GPU, displayed on the other, and the display GPU + monitor has VESA Adaptive-sync enabled, and the game correctly triggers new sync windows as if it had been running on the same GPU.
Does this work in practice? Who knows, but its supposed to, especially with fixes that 1703 and 1709 added.
Multi-monitor setup with 2 on the Radeon (normally all 3) and 1 on the iGPU, seemed to work fine. No real visible lag either, from what I could tell with limited testing.
This means that you have to pay a penalty to use the Radeon to render, but send its framebuffer result back to the HD630 for displaying. This is also the case current Nvidia Optimus or AMD Enduro systems. Draw backs from this approach are frame tear / lack of proper V-Sync and added latency.
If this ends up being used by Apple however, it is likely that a hardware demux will be used to connect the display to both graphics chips, and switch it back and forth on demand. Other vendors don't do this, because it is more expensive, and passing the display from one GPU to another requires the pixel clock rate (among other details) to be shared between the different GPUs, which is not something that the vendor driver (Intel/Nvidia/AMD) stacks support.
Note that such a hardware demux is used in previous generations of MacBook Pros, and it is not publicly documented, which means very poor and/or very late support in Open Source operating systems such as Linux.
Some system builders (Lenovo and Asus have done this, probably others) wire the internal display of laptops to the Intel GPU, and the external HDMI/DP to the discrete graphics. In the past, this allowed you to power more displays (when Intel GPUs were restricted to 2 displays max), and it still allows you to avoid the Enduro/Optimus hassle for external displays, when you are likely to be powering the laptop with the A/C adapter anyway. It also meant that vendor/OEM specific drivers needed to be used in some cases, though.