If you drop the comp in a square pixel 720×486, and then hit Command-Option-F to scale it to fit to comp size, you get the stretched output.
But if you drop it into a comp that has the right pixel aspect ratio (BTW for ntsc widescreen that’s 0,8333, not 0.9) After effects will scale it automatically to compensate for the difference.
the resulting rendered pixels will be the same, however, if you open up the non-square comp you have some extra-features like Pixel Aspect Correction for displaying in the correct aspect.