Not true. The current mini was designed to retain good quality while lowering the cost. Adding retina wouldn't have met that market need. Worse, the power requirements of the A6X needed to move those pixels would mean the thing would suck too much power and run very hot. The current mini actually gets pretty hot despite the die shrink that the A5 went through in the later iPad 2. They'll need to have another die shrink to get the A6X into it and that isn't going to happen overnight. We might not even see a retina mini within the next year and in fact as others have pointed out, the only real advantage the full size iPad has over the mini is the retina display. The other issue is manufacturing the newer panel without defects - the more pixels the more likely you're going to have dead pixels and that all adds to the cost of production.
Comparing the mini to an original iPad or iPad 2 shows a definite improvement in the pixel density. I was shocked going back to the older iPad to see just how obvious the pixels were by comparison, plus the mini screen is a significant step up in quality over the older screen even though the resolution is the same. It looks substantially crisper. More importantly, don't compare a retina device with a mini in portrait mode because you're not seeing the fonts at their rendered resolution. In landscape the fonts look fine, but in portrait it scales the fonts a little since they're rendered at 1024 wide and scaled down to 768 in portrait. It is site dependent though, for instance the mobile version of MacRumors works well in portrait, but the full desktop version is much better in landscape. Apple should really implement minimum font settings in Safari to improve the experience but for everything else, the screen is good and it preserves battery life and performance.