Slack said it had 100% uptime. Did it really?

No, because at 8 nines (99.999999%) you're responsible for the 310ms of downtime per year, aka ping time. 100% uptime would be all of Slack running in memory. See "Latency Numbers Every Programmer Should Know"

https://colin-scott.github.io/personal_website/research/inte...

They probably should have reported 7 or 8 nine's just to not get called out! Technically by their (limited) definition of uptime, Slack is correct. But there's always incentive to underreport breakages, so you want to more heavily weight 3rd party metrics

I think it depends on how you define "uptime"...