Unless they had a way to test if a phone had been hacked and distributed that along with the patch. It's quite possible that they had pretty good telemetry on the extent of the exploit.
I agree that they should have told those who were affected, but perhaps they did?
There are a ton of places in iOs that report back to Apple. When you first set up the phone it asks if you want to "share your usage". You also agree to let them have stats on how long you use each app as part of the app store agreement.
Apple's selling point is that they don't make money selling your private data (or transitive access to it) to third parties, but they don't make any claims about not doing it themselves.
I don't see anything that would preclude them from installing some telemetry for this specific attack. And I think it would be perfectly justified in the name of security too.
I agree that they should have told those who were affected, but perhaps they did?