May I ask what “league rule” this is? And yes Julia Dorsey did play for both those clubs through high school and plays both sports at unc.
Its a rule for the DA Clubs at the academy level, which came from US Soccer. Per their contract, they can't play other sports and can't play for high school soccer team. This is the reason many soccer girls quit club the day after they commit so they can play for their school senior year (and junior year for states where girls soccer is a spring sport.). To be clear, there are some girls who violate these rules. However,per their contract, clubs have the right to drop them for violation of it.
These rules and the entire academy system's rules have changed since Dorsey committed to UNC.
There are exceptions out there. Maddie Dahlien a UNC soccer commit with olympic level track speed is another, but they are all approved by the league/US Soccer.