The article you cited points out this was a rush of people returning books after the amnesty, not a change in ongoing book return rates - as the article says "

The article you cited points out this was a rush of people returning books after the amnesty, not a change in ongoing book return rates - as the article says "The Chicago Public Library system’s 240% jump in book returns may even out in upcoming months, since the new policy change is likely what spurred library goers to take advantage of their newly bestowed late fee amnesty,"