I still don't get why people hate this so much.
The most important fair use factor is the effect on the market for the original work. Obviously if the "Emergency Library" was permanent it would have a major effect, because libraries could buy one copy of a book and lend it to everybody, instead of having to buy enough that everyone who wants to borrow one at once can have a separate copy. But it was never intended to be permanent, and as a consequence it wouldn't have materially affected the number of books that libraries buy.
As far as I know it hasn't even been established that the number of books they lent out was greater than the number locked away in closed libraries everywhere during COVID, and it seems intuitively likely that it wasn't (that's a lot of closed libraries). So the argument is that because it's an emergency and temporary, they can go on an approximation instead of having to do the exact accounting during the emergency situation.
Which doesn't seem that crazy.