The key property for crypto randomness here is that these high energy particle events (be that cosmic rays, background radiation, etc) are not just random, but independent from thermal noise. They are few and far between but they affect each sample somewhere. One way or another all that entropy will get hashed, and having even few bits that are contributed by independent phenomena makes final hash extremely hard to attack.

Considering all sources that contribute noise to sensors (thermal, light photons count, high energy particles, shot/RTS noise, and i'm probably missing a few), all with unique distributions and characteristics makes each sample readout very hard to predict.