A hacker statement (part of which matches one of the forum posts you quoted) was posted on the fastcompany.com site (https://web.archive.org/web/20220928010445/

A hacker statement (part of which matches one of the forum posts you quoted) was posted on the fastcompany.com site (https://web.archive.org/web/20220928010445/https://www.fastcompany.com/90793240/fastcompany-gets-hacked https://web.archive.org/web/20220928010445/https://www.fastc...). The post on the site itself, at least, must indeed have come from the hacker. (Edited my comment with corrections)