I never think you're clueless, but things do change. I worked as a manager at Six Flags 10 years ago, but I don't pretend as if I know how they do things now.
You're right about runners bringing family to help justify the trip, but even so, those people aren't buying as many park tickets imo. I remember I happened to be there last year because today is my wedding anniversary. So, hubby and I forgetting about it being marathon weekend were suddenly reminded when we pulled into a packed Epcot parking lot. I got a little worried, but then remembered they're out running, not hanging out, so in we went and it. was. dead.
So, then we went to check into Beach Club and it was NUTS. The families of the runners were hanging out by the pool, participating in resort activities while the runners rested up and prepared for a park day later in the trip. So, I think you're seeing lighter crowds because the people that are going to stay are going to go to the parks during the week and the locals will have gone home by then. So, while it's packed this weekend, I think come Monday things will be back to normal.
It's also possible since this week had a paid holiday in it people came down to have their vacation BEFORE the races are headed home on Monday.