I just put in a bunch of fake email addresses and they all returned with "Good news - your password has not been compromised." I think the only confirmation that youd get of an existing account is if the password was compromised.
They can modify it to simply say whether your account was compromised, regardless of whether you have an account (ie, if no account -> not compromised).
...Which they ought to do. Offering the ability to enumerate user accounts is unlikely to be the immediate goal of this utility, but it's an effect nonetheless.