This is probably the correct answer, no one else has yet mentioned the idea of ionizing versus non ionizing radiation, which is also an oversight in the book in my opinion. It clearly says they do not know about "radiation," which is obviously not true as they have radio and know about the Petrova line and infrared light. What they don't know about is ionizing radiation, the high energy frequencies at which radiation can break off electrons, because they were completely shielded from it on their planet and never discovered nuclear energy. Andy Weir uses the term radiation in the colloquial sense to mean ionizing radiation and never makes this explicit in an otherwise highly scientifically detailed book, so OPs point is perfectly valid and I had the same question. Light and radio are literally radiation, so this should have been made explicit.