This is not a Windows blog, it's a Skype one. I find it entirely plausible that someone on their web team has been sold on resizing to 50% in the browser, which is one suggested way to engage Apple's spare pixels.
edit: Also, even if it were a Microsoft blog, I still wouldn't consider it an impossibility. There are devices running Windows Phone with Retina-comparable pixel densities, so Microsoft must have considered taking advantage of the super-resolution capabilities in a compatible manner. I don't know if they have done so, but it would be silly if they didn't use any of that spare hardware capability.