The ST bed extender is the same width as the one for an 01-03 Supercrew, but it is about 4 inches shorter--doesn't reach to the end of the F150 bed, so it doesn't sit down correctly. The rubbers on the bottom of the extender that should sit into a ridge at the end of the tailgate actually rest on top of the tailgate and make it sit up a little bit too high to properly align the locking pins into the holes in the side of the bed.
Conversely, the Supercrew extender will physically fit on an ST, although it will sit all the way at the end of the tailgate instead of into the mounting points on the tailgate. But if you have a factory hard tonneau cover, there is not enough clearance (because it is, as noted above, about 4 inches longer) to rotate it back into the bed when not in use.
BTW: this information concerns the 01-03 Supercrew ONLY; the Supercrew of those years used a bed which was different than that of any other model of F150 (and just happens to be the same width as the Trac bed). Standard-bed or Flareside F150's of that period have a different width bed, as do 04-up models.