So just to confirm, you're saying you DON'T think this will leak resin/paint/whatever into the tank?
Well, the resin is apparently "dishwasher safe." That likely means that the resin can withstand the dishwasher and that it's fine to wash it with items that touch food, which says something (i.e., it might be okay to use it). But without knowing exactly what was in the resin, you wont know for sure. Then again, we probably don't know how much leaks out of different plastics that might affect our bodies or those of our animals.Often, things made for reptiles or terrestrial enclosures will have a "do not submerge" note on it. I take it that such a note isn't present.
What the Caudata care articles usually say is "if in doubt, leave it out." That being said, if you can test it by soaking it in a smaller container for a couple of weeks, that might give you more to work with. If it makes the test water smell or discolored, or if it becomes more brittle or pliable than it was when you got it, then you know you probably shouldn't use it.