Yes, and those people didn't get their profile pictures exposed through this phone number enumeration. If they had, then maybe it would have qualified as a security breach.
OK... but it's not phone number enumeration. You need to give it a phone number to check if whatsapp acc is registered for it. So you need to have a collection of phone numbers first. If you have a collection of all phone numebrs in the world then you could enumerate whatsapp accounts.
"The accessible data items used in the study are the same that are public for anyone who knows a user's phone number and consist of: phone number, public keys, timestamps, and, if set to public, about text and profile picture." Source: TFA, which I read.
