The pixel count for a Retina 4:3 7.9” screen would be greater than the 7” Nexus 7’s 16:9 and therefore a non-starter right now for a theoretical iPad mini with Retina, assuming weight and battery life have to remain the same as the current model. It’s what Rene’s getting at in the article for one of his points and I tend to agree with it. However it’s Apple’s own design choice and I guess now they have to live with that self-imposed restriction. It’s not wrong, it’s just that the hardware needs to catch up to meet their requirements and it’s not there just yet.
Apart from that the only thing that should be stopping anyone from buying the Nexus 7 is better tablet specific apps on iOS. If you’ve already invested heavily in Apple’s ecosystem though, that’s another story. However, given that this issue has Google’s direct attention now, it will get better quick like Android phone apps in the last couple of years. And since it’s only 7” and quite narrow in portrait mode, most phone apps scale really well and work almost flawlessly anyway.