Have we considered that Rocky might just have higher levels of lead in his carapace than the average Eridian?
Like, sure he slept near the fuel tanks and I’m sure that helped. But he was up there for decades and definitely moved around the ship away from the astrophage while fixing things, going to watch others sleep, tending dying friends, piloting.
Rocky must be from a region of Erid with more lead in the surrounding area which was incorporated into his body. For Rocky it could have been centuries since he last lived in that place with high lead content yet it still protected him.
This happens with human teeth in that the area where you grow up is imprinted in your enamel for decades afterwards. But that’s not a thing the average person knows. So Rocky never brings it up, because it’s kind of just a fun fact? Like, saying you’re from Oregon in a doctor’s appointment. Ok? Cool? This is about your gallbladder.
If the Eridians didn’t know anything about radiation and Rocky isn’t a biologist or doctor why would he know his leady body is a relevant detail. There’s no reason to think this would come up in the conversation once Grace realizes about the astrophage being nearby. Occam’s Razor: astrophage protects from radiation. Rocky slept near astrophage. Therefore astrophage protects Rocky.
I can just imagine they find this out way later when they’re on Erid and Rocky goes to his hometown to visit his parents and comes back to the bio dome and is like
Rocky: Bring rock from where Rocky from to add to Grace beach! Has same texture as Rocky! Same same same
Grace: cool! Let’s put this baby under the microscope. We can shoot it with the element detector!
Rocky: yes yes yes look at rock! Hear hometown sounds.
*Several experiments later*
Grace: uhhh. You might want to sit down.
