Don't get me wrong, Zendesk definitely has their own separate problem: you should not be able to CC yourself onto an existing support ticket by emailing a guessable ticket ID.

But simultaneously you should not be able to get into a company Slack by simply having an account with a @company.com email address created by a third-party SSO provider.

In other words, even in Zendesk fixed their problem, Slack would still have a problem on their end.