I wonder if "1%" is just a number that is repeated when talking about this subject. Another article debated on HN:
https://rantt.com/the-paradise-papers-how-ridiculously-easy-...
also refers to "the 1%".
There's a detailed article on the 1%:
https://dqydj.com/top-one-percent-united-states
EDIT: It seems to me that 1% is beyond accessibility. I think that 10% is a (very generic, rough) threshold that can be reached with effort, but not 1%.