The implication with both this statement and the map is that we should be comparing the total number of people who died from radiation, and the total from fossil fuels, and see which is bigger, but there are other ways of evaluating (potential) harms.
Nuclear power and other radtech is not equally common around the world, and neither are fossil fuels. In the context of promoting transition from one to the other, the important question is the relative harm of these two choices as a function of their deployment over time. There have been events from minor leaks to world-changing disasters, all of which are contingent on human factors which vary widely across time and space. So it's not clear that increasing global rollout of nuclear power will be as consistently safe in the future as it has been in the recent past.