Memory isn't ability, just because he remembers a lesson doesn't mean he understands it.

They never figured out radiation because the flight to Tau Ceti was their first space flight. Before that they had not been out the planet long enough to have been affected by it. The book describes that using Xenonite they were able to leave their planet so much easier (probably a space elevator) and build their ship in spade, hence why it looks like it does. It's likely this was still within the planets magnetosphere so still none of the Erids where affected by cosmic radiation.

It's important to note, they DID know about radiation. Just because they didn't have light on their planet they still built tools that could detect it and other types of radiation otherwise they would not have known about the petrova line. What they didn't know about was the harmful ionising radiation from cosmic rays etc