Apple does apply gamma correct blending, at least when rendering text to a solid surface so that subpixel alpha kicks in.

If you render it to a transparent layer and force an RGBA blend, it gets blended in sRGB color space rather than linear RGB, but this is a flaw that can't be easily changed because of legacy reasons. Any design that uses transparency of any kind, text or graphics, would change appearance. I agree that we should have a body { color-space: linear; } though, but good luck in explaining why to coders and users alike.

The fattening that you are complaining about is something else entirely: it mimics the way physical ink bleeds on paper. The weights were under designed to compensate for this, and they replicated this effect virtually.