On the same motherboards? That's rather impressive, if the same motherboard can support two different chip architectures. Having an ARM chip that just happens to use an AM3/AM4 socket is less impressive.
I'm purely a layman in this space, but I don't see why it would be impossible. Pretty much all modern CPUs are actually SoCs. Modern peripheral connections like PCIe and USB 3 have direct access to the CPU rather than going through custom bridge chips, simplifying board design and making it possible to use the same board for two architectures. Audio and network chipsets don't care what architecture the CPU uses.
Again, I'm far from an expert but it seems like a simple enough solution.
As someone with a lot of experience in board bring-up, trust me, the hard part isn't grabbing AMD's reference board, sticking on an audio chip and a wifi chip and sending it over to a board manufacturing shop.
The hard part is wrangling the software: BIOS, drivers, testing and more testing, patches, microcode updates. Finding out that your wifi chip vendor has moved the chip to end-of-life just before you were ready to ship would be a non-issue if the software part were faster.
