The Slack takeover exploit is a problem on Slack's end (and sounds more like a configuration issue than a bug) so Zendesk would not be responsible for that anyway though.
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.