It will be close, but in comparison to the surrounding retina (eg. @2x) elements, the lower resolution will look blurry, as it is being mathematically upscaled.

I've never understood this. Since on the hardware level it's exactly 2x, so one pixel becomes 4 pixels.

Why on earth would it look blurry? There is no interpolated upscaling needed.

He's saying compared to retina elements visible in the browser chrome or (on a laptop screen) in other applications open at the same time. If everything on both the retina and traditional screens was not retina optimized than I can't imagine there would be any noticeable difference.

Yes, this is a very real problem.

I’m working on a project that involves CGI animations, and we’ve had considerable debate about what resolution(s) to offer for the video files. On a desktop or laptop, a size similar to YouTube works fine. On an iPad with a Retina display, the videos can sometimes appear to be of a lower quality than they really are, because we’ve got sharper icons and text nearby.

Having said that, it’s not so much the video itself that can look awkward (since people are used to somewhat lossy video compression on the web anyway, and in motion you’re not really seeing the individual pixels) but rather the static poster image that displays until you start to play that video.