What they probably mean is that it's non-hazardous for the use it was designed for. I doubt that they intended it to be fully inert in water, which is the measure of a food safe item. polypropylene IS usually considered safe, it's the chemicals that it was prepared with that can be harmful. For example what chemical was sprayed into the mold to make the foam not stick when it comes out of the mold or sheet that it was expanded on, etc. The only way to check 100% is to contact the manufacturer. IF that is unrealistic, you can at least soak it in water, weighted down, with vinegar, isopropryl alcohol and propolyene glycol in it. The vinegar and alcohol will help lift away any chemicals. The propylene glycol is a food safe chemical used in food colors, it has the effect of dissolving hard to dissolve substances like the dye in the food color. It is particularly effective when used to clean polypropylene. If you find it in a large enough amount you can use it at full strength to wipe away any chemicals on foam, new acrylic or plexiglass, newly hardened epoxy or resin used to make wood tanks, etc. I have used propoylene glycol to dissolve flubendazole or metrodaizanol medications that normally don't dissolve in fish tank water to make them more absorbable biologically to the fish. It can also be used in preparing new driftwood for aquarium use.
thanks krusty! 
