Is anti-aliasing color processing correct in this image?
The anti-aliasing rendering is correct.
The reference gray ramp (top and bottom) is linear in sRGB color, not in brightness.
The second gray ramp made with anti-aliasing is linear in brightness. The rectangle color (black in color sRGB space) is mapped to linear space (remains black), then anti-aliasing is computed, and the resulting image is converted back to sRGB. It is thus not linear in sRGB value anymore.
The RGB values given by a screen color picker are in  sRGB color space, and the color values of a screen capture are in the sRGB color space.
SVG and nearly all browser graphic library (skia and cairo) are doing it wrong. They compute anti-aliasing over sRGB instead of linear color. They thus don't convert back from linear to sRGB. The same mistreatment  is also visible on Windows, MacOs and iOS.
The filled rectangle gray ramp doesn't have this problem because the conversion from sRGB to linear and its inverse cancel out.