Comment by modeless

The model specifically said that the material is less dense than water. If you said that the material of a balloon is less dense than air, very few people would interpret that as a correct statement, and it could be misleading to people who don't know better.

Also, lighter-than-air balloons are intentionally filled with helium and sealed; rubber ducks are not sealed and contain air only incidentally. A balloon in a vacuum would still contain helium (if strong enough) but would not rise, while a rubber duck in a vacuum would not contain air but would still easily float on a liquid of similar density to water.

The reason why it seems like a nitpick is that this is such an inconsequential thing. Yeah, it's a false statement but it doesn't really matter in this case, nobody is relying on this answer for anything important. But the point is, in cases where it does matter these models cannot be trusted. A human would realize when the context is serious and requires accuracy; these models don't.