You are right. But I think parent comment understands the concepts, and was misled by the name, which to be fair, is a misleading name. The names "subpixel AA" and "grayscale AA" are both terrible names.

Grayscale AA isn't somehow gray or monochromatic, it is regular anti-aliasing, it works in color, and it works by averaging (virtual) sub-pixel samples. The name is trying to refer to how it's not making use of the physical LCD color subpixels.

Both names assume that the term "sub-pixel" is primarily referring to LCD hardware, rather than general sub-area portions of a pixel. From my (graphics) point of view, the LCD case is a specific and rare subset of what people mean by "subpixel", which is what makes these terms seem misleading and awkward.