Plutonium is astonishingly poisonous. But conversely, the amount of Pu
Pu used by a mission is fairly small. And it naturally decays.
Comparing double digit kilograms of horribly poisonous stuff (say 10Kg10 kg), vs hundreds of thousands (if not millions) of kiloskilograms of propellant (even Tetrazinetetrazine, MMDH) makes it seem true.
Additionally, knowing how bad the Pu
Pu is, it is surprisingly armored. People forget that Apollo 13 dropped the LM's RTG into the deepest part of the Pacific, and the belief is that the containment vessel survived one of the fastest reentries to Earth's atmosphere. (I like how Top Gear America's first driver on their track was 'the fastest man they could find' Buzz Aldrin, but of course the Apollo 13 had him beat by a bit).