But it's not the gamma rays that are the reason for the systems redesign, it's the fact that "at least one of the pilots who flew the scenario in a simulator found it difficult to respond in time to maintain control of the plane".
Boeing's assumption that pilots could respond quickly enough to a loss of control in the cockpit was an underlying motivation for designing MCAS with inadequate safety measures.
That's why they have to rearchitect the system. If those bit-flips happened to the Flight Computer in command, they could lose the plane. Therefore, redundancy must be built in.
It wasn't originally evaluated as posing a catastrophic outcome in the safety analysis, which is why no one paid it any mind.