One thing to keep in mind is that the outage did not affect the whole Cloudflare infrastructure, only part of it.
If you UptimeRobot monitor was hitting an unaffected datacenter, it would have no reason to send you an alert. You’d need to set UR or a similar service to monitor several CF datacenters (at least the ones most likely to serve your website core readership) to be able to have a clear picture of its availability at any given time.
I did receive an UptimeRobot alert for a Keyword Monitor related to SSL certificate. I have 3 domains with the same monitor, but only one was triggered. (The one for which the alert was triggered is a
.com.br domain, while the other two were
.com domains) However, when I used my cellphone to visit this domain it was available from my location. So I guess UR sent its monitoring request to another of CF datacenters, and hence the discrepancy.