[Johnny555]

I think Slack did not actually know how to handle it: they outsourced the handling of this; they passed the buck

The issue was a transit gateway, a core network component. If they weren't in the cloud, this would have been a router, so they "outsourced" it in the same way an on-prem service outsources routing to Cisco. I guess the difference is they might have had better visibility into the Cisco router and known it was overloaded.