No, they are not. First of all, none of AMD GPUs can handle 5K resolution. Secondly, ONLY Nvidia Maxwell can even run that type of resolution, on single display. ONLY Maxwell cards have HDMI 2.0 and DP 1.3 that can handle that resolution.
3rd. Top end single GPU from AMD costs over 500$. Nvidia GTX970 costs 200$ less, and is on par in terms of performance. Seocndly, R9 290X draws over 270W of power, the Geforce from Nvidia draws almost half that with 142W.
I think the whole article is BS in consideration TECHNOLOGY and Pricing right now.
Only AMD card that COULD possibly handle 5K display right now is few months to come, and that is AMD Fiji GPU.
If we are talking about 4K, then simple integrated GPU from Intel can handle double resolution from 21 inch iMac. But for 5K, iMac MUST have Nvidia Maxwell, or AMD Fiji. Maxwell are right now. Fiji will be next year. If Apple will show 27 inch 5K Display for their computers, whole lineup including Mac Pro must have Maxwell GPUs.
Unless they go with the iPhone 6 Plus/rMBP approach with a scaled resolution. You'd have a native 4K iMac just scaled to "look like 2560x1440" so that you get a higher ppi with the same screen real estate on the 27". So off-screen render at 5K (5120x2880) but the display itself is not native 5K.
That said said, a 27" 4K native iMac would have a 163ppi density. Would Apple consider that a "Retina display"? Based on distance from screen, I'd say they would.