It's where a lot of tech employees hang out. You have to verify that you have an email address with the company's domain to post, but otherwise super anonymous. It can be super toxic and you see a lot of fake posts, plus your account isn't revoked after you leave the company, but the original OP definitely has/had a Twitter email address.
Blind is an app that provides an anonymous forum and community for verified employees to discuss issues. Users on Blind are grouped by topics, company and their broader industry. The app verifies that the registered users actually work in the company through their work email and claims to keep user identities untraceable.