This is not quite accurate. No school is required to pay anything if they opt-in, it just gives them the ability to do so. I would guess most, if not all, will to some degree. Very few outside the P4 (and maybe even some on the bottom end of that) are going to get anywhere near $20.5 million (or 22.5% of revenue).
Not sure if/how the 22.5% of revenue part effects UND as if it is tied to a portion of total revenue, they would be capped at much lower than $20.5 million (which they wouldn't come close to anyway).
There are also new increased scholarship limits in other sports that will have to be funded to remain competitive in those sports.