I don't think that the sanding sealer is just watered down Enduro. First, they actually make and sell a thinning agent. They do not say to thin with water. Second, the sealer can be applied under several different types of topcoats as well as finish sheens. Are they saying that the sealer is thinned Gloss Poly, Eurethane, or Acrylic? It can be used under all three or even any of their waterborne colored finishes according to the site.
Just too many unanswered questions IMO to accept that it is just "watered down Enduro".