To try and give the simplest answer.
The vendors firmware doesn’t just include android, but also the boot loader which is made up of a couple different binary blobs.
Besides handling booting the device, these bits also handle some hardware access for the running OS, and stay running in a separate secure thread on the device. The kernel talks to this thread, and makes requests of it when it needs certain things.
This is were the the problems with old firmware come into play. The kernel expects to ask for stuff in a certain way, and get a certain answer back. As development continues some of this ends up changing, either to fix issues or to make things more flexible or efficient.
Normally when a vendor updates firmware, they update both the boot loader and kernel at the same time, so there doesn’t end up being any issues. This hasn’t been an issue for CoreELEC in the past because the 3.14 kernel was just as old as the old firmware. But, the 4.9 kernel is much newer, so there is a chance of issues with older firmware, and having a miscommunication that causes a problem. I have patched a few such issues, and will try to fix more of them when possible.