'After the fact' may be of only limited legal importance in assessing the relative values being weighed in the case.
Let's say that for book X, IA held-in-reserve 2 copies. And at the peak of the "Emergency Library", they never loaned out more than 10 total copies.
I'm sure publishers would allege: "You created 8 net copies! Pay us!"
But then 500 physical libraries who are sympathetic to the IA show up, and say: "Amongst us, we had 1000 idle copies of X trapped on our shelves. We wish we could have done the same sort of 1-for-1 lending IA did, to help the nation in its hour of need, but we weren't set up for it, and couldn't throw it together with limited resources during mandatory-closure orders."
"So, as soon as we heard about IA's program, we endorsed it & recommended it to our physical patrons. As far as we're concerned, we delegated the right to loan our copies to IA, including retroactively."
"No net copies of X were created. Rather, if there had been no pandemic emergency forced closure of physical libraries, our copies of X would've been loaned out for a total of 40,000 library-patron-days. None of that happened, but IA's emergency lending managed to reclaim, according to their exact usage numbers, 5,277 of those days-of-use for students, scholars, and citizens. Less than 15% of the pandemic-destroyed-value."
"Now, publishers are trying to profit off the pandemic, by not only having collected their payments for all those idled books, but collecting a windfall of extra purchases, and/or damages, from a non-profit library that was just trying to offset massive pandemic losses in a technologically efficient way."