The website haveibeenpwned says the original breach date was 18 December 2020, with the information becoming available on 18 July 2023, with a total of 3,943 compromised accounts. The site notes that as well as all the above information, the leak even includes each individual’s t-shirt size.

Looks like someone in charge of organizing that event got phished. This seems like the type of info that can fit into an excel sheet.