It say it "softens the entire picture and has a low performance hit"
Yeah, post aa, in an attempt to get rid of jaggies at the lowest performance hit possible, will affect the entire image, rather than just the edges in question.
But this has a side effect of blurring textures aswell as the edges that require it.
MSAA will soften only the jaggy edges without affecting texture sharpness, but obviously has a bigger performance hit.
Just using MSAA 2x with Post AA OFF will give you better image quality.