The article isn't clear on if the credentials were theirs or not, or how Rockstar management obtained them. It is possible that Rockstar management simply signed up for a Discord account and went through the process to request access, which was then granted to them by the Discord server admins. Since the server admins used the company Slack to verify that folks requesting to join were employees, it seems quite plausible that is what happened. But the details aren't really there in the article.