Read Van Allen's article on the radiation belts in the 1959 Scientific American.
Done that. It closes with a summary, handily highlit in yellow in your version.
"Our measurements show that the maximum radiation level as of 1958 is ... Since a human being exposed for two days ...."
First - for every MAXIMUM level, there's a matching MINIMUM, is there not? If the maximum level is X, the level EVERYWHERE ELSE must be less than X, must it not?
Second - Apollo astronauts were not exposed to even the maximum for two days. They were exposed to a far lesser level for a matter of minutes.
You do realise that further studies were done after 1958, don't you? That this wasn't the final word?