Wow! I always assumed it worked the way @Binkley described. It looks like a more complete description is that every tied player gets the points for the average position among tied players; use the lower (better) position number in the case of a tie between an even number of players.
So a tie for 11-12th is 11.5, which becomes 11(yay! the tournament just awarded more WPPRs than expected). A tie for 13-17th becomes 15th; I’m not sure if this is more or less or the same overall WPPRs for the event because the dynamic distribution of points for everyone other than first place.
Thanks for pointing that out @JShop.