I think it probably is a matter of the custom hardware that Apple used to enable the 5K display -- it's not worth AMD's time and expense to write a Windows driver that handles a chip only found in one model of PC (the Retina iMac) made by a single vendor, especially when many of those machines will never have Windows installed in the first place. And it may not be worth Apple's time and expense to write a custom driver for Windows which supports the 5K resolution, when the stock AMD drivers (minimally customized to recognize the M290X/M295X) support 4K and it probably won't make a huge difference to most of their users. And after all, it's Windows, so not their biggest priority.