I would guess the main value is in practicing the technological skills of sending a thing to the moon and maintaining connection with it.
Are we able to also simulate the radiation in a laboratory?
I will wager that this was done prior to launch, and current longevity is not a complete surprise.
As for radiation, I attended a lecture series last year about the various kinds of radiation testing involved in designing hardware for satellites, they probably did do similar things to those for satellite electronics (stuff like firing beams of IIRC electrons over the PCBs to see what parts might need reinforcement/redundancy against random charged particle strikes).
Also, the point of SLIM was to test the new automatic landing system, a combination of camera-based navigation and a new landing strut system meant to handle inclines. 100 meter precision instead of previous 20 km. Everything that comes after that is near-free extra data.
In that documentary, it was remarked by one of the engineers that scientists often designed these instruments to be more robust than what was expected.
https://www.jstage.jst.go.jp/article/tjsass/66/6/66_T-22-48/...
So ya, they probably didn't do any significant testing below 0C.
There's another problem here: below a certain temperature semiconductors become insulators. You're running the risk of your chip shutting down in a disorderly manner.
−273,15°C < −247°C.