"Breached" implies that sensitive data, like payment details, private communication, or physical addresses, were leaked. Instead, this is just semi-public stuff like email/username/name. Maybe a better title would be "15M Trello users have been identified (name/email)"
Funny. Back in my l337 days public Trello boards were one of the easy ways to get passwords. People would put shared passwords for team accounts just on their board, in plain text
That title is very misleading. 15M Trello accounts were found to be compromised because of other, previous leaks, but no leak related to Trello occurred.