That is like 17 of such 15 minutes breaks per year, i.e. an allowance for one small (or a large fixed quickly) screwdup/month :)
Today's incident did not impact App Engine at all.
(Disclaimer: I work in Google Cloud Support.)
"Downtime Period" means, for an Application, a period of five consecutive minutes of Downtime. Intermittent Downtime for a period of less than five minutes will not be counted towards any Downtime Periods.