This is patently absurd. Those exposed to the most radiation are all dead and died from radiation exposure, effects, and side effects.
The idea is that a small amount of radiation spread over time is survivable. So in sum total he may have had more than anyone else. (I won't include anyone who instantly died in hiroshima.)
My initial thoughts went to Marie Curie (small amounts over time), and Harry Daghlian and Louis Slotin, or other Los Alamos engineers involved in criticality accidents.
Plus, other clean-up workers working in disasters like Chernobyl whose deaths from radiation caused illnesses most likely went unpublished.
So at best, that puts him in 10th place with a very loose interpretation, or otherwise significantly behind, and maybe in the top 10 of living individuals. The phrase "in history" is poor writing, however.