The point is to use it as a sealant to make custom decorations, not really to seal a hole.
Petstore decorations seem to be mostly ceramic, but many paints made for ceramics would be very toxic in an aquarium setting, the paint would peel and get in the tank and dissolve. Not good. Those decorations are sealed--with something. No one has been able to give me a clear answer on what they are sealed with that makes them aquarium safe, so I thought--maybe an aquarium sealant used on top of decor would separate possibly harmful chemicals and materials from entering the water.
Part of my inspiration is that people create custom reptile enclosure decor all the time. Greatstuff is a foam that can be used to carve into and create fake rock walls. For reptiles, you would paint it and then seal it with something like concrete or grout. But they aren't as sensitive as bettas--(physically, as the hardness of concrete/grout could hurt their fins, or chemically from anything possibly leeching through)
Thank you for responding though! I guess--to anyone who's used aquarium sealant before. I know it can stop water from leaving the tank. Can it stop stuff from entering the water just as well? How could I test this before putting my betta in harms way? What tests should be preformed and what might change in the water quality if it does fail, so I can keep an empty tank and get a 'heads up'?