Just found out that they are not.
The fleece flippers internally use coroplast which is not safe for chins to digest. It also consists of Uhaul recycled blankets which are made of recycled denim material. The fibers aren't like that of fleece.
Though- in theory, if the fleece flippers fit in the cage correctly, any access to the internal system would be quite hard to get to for a chin. It would require them to lift the sides up, and flip it to the shorter sides to get inside of the fleece flipper. I don't know, in a normal cage setup, if that'd be realistic and possible for a chinchilla to do so.
We could, instead of using the uhaul blanket, use cardboard instead of coroplast, and wrap that with washed fleece? It's the only option I can think of, but I don't even know if it'd have the ultra absorbent layers that the uhaul blankets do.
That's my research. haha.