This is untrue, the front sight would not appear blurry in 1x mode. Parallax is to do with the appearance of the reticle depending on the viewing plane. A parallax free optic uses projection methods to make the reticle move as little as possible in relation to the target when being viewed through the optic off-center. The issue 1x-Nx optics have is that in 1x mode is that the user must look directly through the optic as they would with a magnifying optic, otherwise they will experience scope shadow and a potentially misaligned reticle.
Example:
http://www.ar15.com/archive/topic.html?b=3&f=18&t=454478