Let me explain it this way: Because of the soft cap, you can't sell all seats beyond 20k. So let's say for 10 bleachers you build, you can only sell 2. Those 10 bleachers cost $2000, that is the money you invest. You only sell 2 bleachers, so say at price $10, you earn $20 more than you would normally without the investment in the extra seats. Now, after 100 home games you will have earned 100x $20 more, so $2000 more than you would have normally, earning back your investment. After those 100 home games, you will have earned money on your investment. It's slower because normally you earn back investment in the arena in 1,5-2 seasons, but still there is a payoff after some time.
So that was what Manon was saying, if you're in for the long run, you can make money by building beyond the soft cap.