It's like the difference between short selling (legal) and naked short selling (illegal). The reason the latter is illegal is because it involved acting as if you owned something that you didn't actually own.
If the IA wanted to treat other libraries' books as its own for the purpose of CDL, they should have (temporarily) acquired a right to do so by entering into an agreement with those libraries.
It's crazy to think that any one entity can unilaterally claim that it is the conduit for distribution of the titles of other libraries besides its own, shuttered or not.
My library is closed between 5pm and 10am every night, and twice a week during the day. Should the IA be able to temporarily claim my library's inventory as its own during those periods?