Thank you for pointing this out. I updated the essay to mention how long the pause took explicitly:
After about a 3.5 second pause [^13], the failover function completed smoothly! We had a new Postgres instance serving requests, and best of all, nobody noticed.
[^13]: About 2.5 seconds to let active queries complete, and about 1 second for the replica to catch up
If there is not noticeable user impact or unavailability of services (this is unique to each service in existence) then there is no downtime.