They should be able to explain why so many people received the email though. Was there a fault in the notification system or not? Are they going to send messages to the individuals which received the notification in error?

I get that direct evidence of a leak is difficult. However, a sudden surge of master passwords being known by third parties in uncorrelated accounts is a very good evidence that something happened. If that's not what happened, then what happened exactly? Was it really a bug in the notification system? Do they have evidence that the password used in the blocked login attempts weren't really the actual master password?

There is a lot of things they can do to show they are on top of things.