So they have a operable spaceship, they know what radiation is because they talk about cosmic radiation and shielding, but they don't have any way to measure radiation or awareness of what exposure looks like or even an inkling that it could exist on a planet, even after visiting several defunct technological civilisations. It's a fun story, but it goes to show the lengths you have to go to to contrive a story where there's a possibility of harm to even a couple of people.
If nothing else they appear to have microelectronics (e.g. video playback and displays). Manufacture of that requires knowledge of radiation because you need to keep the encapsulation material low-activity. Plus the general ideas behind semiconductors and microlithography are based on physics very close to the physics of ionising radiation (e.g. photoelectric effect) and you probably use D/EUV, X-rays and ion/electron beams somewhere in the process too.
If nothing else they appear to have microelectronics (e.g. video playback and displays). Manufacture of that requires knowledge of radiation because you need to keep the encapsulation material low-activity. Plus the general ideas behind semiconductors and microlithography are based on physics very close to the physics of ionising radiation (e.g. photoelectric effect) and you probably use D/EUV, X-rays and ion/electron beams somewhere in the process too.