The sad part is that on ARM the kernel is usually also custom compiled for the board. So what happens is that Armbian ship a different image for each board.

If you go and look in https://github.com/torvalds/linux/tree/master/arch/arm you see a zillion "mach-xxx" directories for different SoC architectures, even if they all use Arm.

Device-tree is a partial solution, but no-one seems to have an incentive to finish the job and let a single image run on any (sufficiently recent) arm board. It's difficult for the community to fix because most people have only their own board. Someone would need to pay for a CI rig with every board, and some kernel devs to do the work of building a single kernel to run across everything. (I think that's originally what Linaro was for - not sure why they didn't finish the job)

Good, you understand the situation perfectly.

> And (assuming you're correct), how does Armbian deal with that / get around it?

You'll notice that if you try to download it from https://www.armbian.com/download/ , nearly every board has a different download image; this is because every one of those images embeds its own boot chain. There are efforts (in some projects, I'm not aware of armbian doing this) to build some amount of early bootloader per-board (often uboot), and just make the install steps something like "install this per-board thing, then install the real OS using a standard image" but that's less common and doesn't work super well when that initial bootloader has to go on the same storage device as the main OS.