I think it's a quite fair assessment. It's not an indictment of their engineering or anything, but they can't say for sure what caused the issue and the analyzed all they could. The conclusion is "we don't know, we have some guesses". Probably it irks me the most because "cosmic rays" are impossible to prove. It's the perfect scapegoat. If I had a penny every time that someone put it out as the possible cause of a bug... I'd still be poor, but... well, I'd have a couple of pennies.

EDIT: On a deeper read, I am inclined to be a bit more charitable to this theory because, to my surprise: "As noted in section 3.5.2, the CPU module on units 4167 and 4122 did not incorporate EDAC"

I did not consider these units were _this_ old, so they did not have error correction on them. Nowadays, most every MCU has ECC on them. So, yes, without ECC the odds are quite larger that they DID get a "bit flip"