First, I believe Apple is working on adding retina displays across the product lines. Macs will still support non-retina external monitors, which are still common. The 27-inch iMac might be a 4K or 5K panels, we will have to wait and see.
Where I think DigiTimes is wrong is on the details. They hear a rumor and embellish. Sometimes they're right, sometimes they're wrong, sort of like a coin toss.