In these specific instances I agree with you. The few I looked at were just single or no post communities. I think the mods just want to give these a chance in case anyone actually does want to take them over and breathe some life into them.
In general though, I think it’s healthy for the fediverse to have options, even with similar topics. I’m sure you might recall situations on Reddit where people didn’t like how a sub was moderated, so some people would set up r/TRUEwhatever.
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/