I think we may have too many senses for the word "design" floating around because this remark reads like an oxymoron: "This is not a design deficiency of the typeface. It was simply not designed to work as body text for big body text sizes and dense screens."
I like the idea of "grades" because it seems abstracted somewhat from the device's details, but the fact that Retina displays are different in portrait and landscape mode is worrisome. I wonder how this concept is going to scale as new devices with weirder resolutions come out.
I think the two senses of "design" are confusing the issue. If he's absolving the typographer for not accounting for retina displays, fine, but it sounds to me like he's absolving the typographer for making a poor decision with an after-the-fact assertion that it was intentional. I don't think intending for something to suck is an adequate excuse.
Title fonts can be a lot weirder than body fonts. If tweaking it to look good at smaller sizes made it look crappy at large sizes, that was an acceptable tradeoff at the time.
Modern fonts, particularly at least OpenType, can embed different information for different sizes pretty easily. If this problem could be solved by using a more modern font format and better quality fonts, that seems like a better approach.
Also which subpixel arrangement is in use. And ambient lighting conditions. And (and this is a big one that no one seems to talk about) reading distance.
The choices they posit, for precisely the same LCD panel are going to change depending on whether it's held in the hand at 20cm or stuck to the dashboard of a car 70cm away.
Basically, I don't think this is solvable. It's cute, and within reason (e.g. for an application designed to read prose text on a known set of devices) maybe it's worthwhile. But the problem is just too hard for a general solution.
I like the idea of "grades" because it seems abstracted somewhat from the device's details, but the fact that Retina displays are different in portrait and landscape mode is worrisome. I wonder how this concept is going to scale as new devices with weirder resolutions come out.