He's actually talking about tickets sold. I think there were only two real sellouts last year(>95% tickets sold). But it doesn't make his point correct. What you're seeing with attendance recently is NDSU finding the sweet spot between supply, demand, and price. With a fixed supply, NDSU has been raising the price of seat fees until the demand of people willing to pay the higher prices equals the fixed supply. That way they're not leaving money on the table. Economics 101.
In a new stadium*, the supply would change, so you'd also change the prices. The top prices would stay about the same because there are only so many seats between the 40s and low down. But the mid-range prices would need to come down, and lots more low-price seats would be added. This would bring large numbers of priced-out fans back to the games, and a new supply/demand/price balance would need to be ironed out. What would the final demand be? Don't know. But it would easily be in the low-20s with lowered or no seat fees in the endzones. Remember that there are few if any FCS schools that have higher ticket/seat fee prices than NDSU; and not all that many G5 programs either.
*No, I don't have a clue where the money would come from. This is just a hypothetical to show the point.**
**This is also assuming an enclosed stadium as a replacement. Go with an outdoor stadium and the demand drops through the floor over the long-term. Season ticket sales suffer because people don't want to commit to an entire season when half of the games are miserable. And without season tickets, there go the seat fees that are driving the money bus. At a guess, I bet with an outdoor stadium you'd have barely larger average attendance than now, but with a big drop in overall revenue.