That is not true. There was hours before they started annotating any kind of service issues. Maybe from when you noticed there was a problem it appeared to be quick, but the board remained green for a large portion of the outtage.
1 hour (52 minutes according to the article) sounds meh. I wonder what their error rate and latency graphs look like from that day.
They've discovered it right away, the Service Health Dashboard was not updated. source: link.
But yes you’re right, there’s no reason to question their monitoring or alerting specifically.