Not necessarily to justify Gizmodo in this instance, but Slack does paywall their SSO feature behind their Business+ Plan, which seems to currently run $12.50/mo/user, which is about a 70% increase from their next pricing tier. See: https://slack.com/pricing

Given the price difference I wouldn't be surprised if they didn't want to pay for that.

Edit: someone later in the thread linked this page which helps explain why this is generally a bad practice https://sso.tax/