... but I do question why a single pixel of anti-aliasing used to define a shape or curve that would otherwise be impossible at a given resolution can't be considered part of a cluster?
I just tell people that we pay more attention to the pixels when they are bigger, and less attention to how everything meshes together into a whole.
True enough, but I see heavy use of 2x2 pixel squares in Helm's star girl wip, and that is effectively a bigger pixel, but that apparently counts as a healthy cluster by his definition. I'd contend that two diagonal pixels with anti-aliasing to fill the 2x2 square would in most cases look more natural at any resolution than a solid colour 2x2 square. It's particular noticeable in the rib area. Also, I'd say in his effort to avoid using single pixel diagonals he's created more noticeable banding on her waist area, above her hips. Particularly on the left side of the image. It still looks great as a whole, but I'm not seeing how it is improved by a refusal to use single pixels or single pixel diagonal lines. Honestly some of the finished areas kind of look like the second last step in a progress gif before the anti-aliasing is applied.
Another thing I want to bring up is the indents in the door in Cure's mockup. I'd argue that their entire structure (2x2 square hugged by a 3x1 reverse L) is perceived by the eye as one 3x3 square cluster with the shade of the door in two corners acting as single pixel anti-aliasing to make it appear more unified at native res. As soon as you zoom in those areas kind of fall apart. I absolutely think that those indents would look much better when upscaled if they used an extra shade to AA the square corners.
I guess what I'm getting at is I don't think a cluster should always be defined as a group of pixels of the same shade, but rather as a group of pixels that are read as one clump.
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Edit:
Here's an example of an edit I made (top) to a sprite Slym made that I think illustrates my point about single pixels appearing to be part of a cluster of surrounding pixels that still hold up once upscaled:

Somewhat ironically the edits I made are mostly in line with the principles outlined in this thread before I had even encountered the concept so I seem to have been naturally incorporating it into my work for some time without concious knowledge of what clusters were in pixel terms. The only real change I think I would make after reading this thread is to make the two pixels standing vertically directly above his eye the same shade rather than two.