Absolute nonsense. I'm sorry but it is. Aside from anything else the A6 is running on a 32nm fabrication process that Apple only started using in shipping products last April (the A5 in the Apple TV was the first). It simply wasn't ready in time to be fully tested and deployed on the A5X otherwise Apple would certainly have used it (the 45nm process on the A5X is the last thing in the world they'd want to use if they had another option as it results in a large, expensive part that runs hot and limits clock speed). The A6 is a custom design realised in part thanks to that 32nm process so even if the A6 was done back in January (remember it has to be done and signed off in time for Apple to build the products, not for the shipping date) they couldn't have rolled it out.
Sorry but I'm getting really sick and tired of this sort of tripe. The A6X is pretty much at the cutting edge of mobile SoC's, the display is class leading, wireless support is as comprehensive as you're likely to get... where exactly is Apple behind the curve when it comes to the iPad?