It’s sub pixel in the respect that it independently uses every controllable dot on your display (three for every pixel, red, green and blue) to create the illusion of black lines. Zoom in on the font and you will see that no pixel is actually black or grey. It uses the underlying hardware structure of the pixel display. It basically doesn’t manipulate the abstract and perfect pixel (throw three colors at a small square and get the combination of the three perfectly distributed across the surface of the square) but the underlying hardware structure of pixels. That’s the reason why it breaks on different or rotated hardware.

It means that the anti-aliasing is done with the positions of individual RGB components in mind instead of treating whole pixels in a grid as the fundamental abstraction.

It allows for smoother anti-aliasing at the cost of some portability (things change if the display is rotated, not all displays have the same sub-component configuration, etc).