1. Zendesk allows you to add a CC to any existing support ticket by sending a (spoofed) reply from the original requestor's email address to that ticket's Reply-To address and including a CC in the email.
In some circumstances the Reply-To address is based on an auto-incrementing integer so it can be guessed. (Although this may not alway be the case: my email archives show some emails from Zendesk using the integer and other emails using a random alphanumeric string. It seems to vary by company so it might be some sort of configuration setting?)
2. Slack allows third-party domain-wide logins via Sign in with Apple without additional verification that the email belongs to a real person. Here the author of the article pretends to be support@company.com to Slack and Slack lets them into the company.com channel, despite the fact that support@company.com does not actually represent a real user and is only intended to be a receiving email address that forwards into Zendesk. (This sounds more like a configuration problem than anything else though.)