The number is from a very rough estimate that's cited on an old page on CDC's website:
https://www.ncbi.nlm.nih.gov/pubmed/21342903
The reasoning is stupid. That pandemic is one reason why nations are trying to be more proactive now. Another stupid thing about this line of reasoning is that there was a strong and widespread negative reaction at the time, among the public and in the media, but nations did not go to sufficiently great lengths to slow down spread - mostly for understandable reasons. We're more aware now, even if Lewk's 30-y-o Boomer brain has turned into goop.