Cannot remotely access PostgreSQL on EC2 instance
(self-answering in case someone else has these same symptoms!)

Then I stumbled upond Shubham Dipt's post on
"PostgreSQL on EC2 (Ubuntu) in AWS",
making me aware of the "Security group" layer AWS maintains.  It's
easy to specify a new one  via the AWS console, specifically adding  Custom TCP: 5432, Source: Anywhere and SSH TCP:22, Source: Anywhere rules for Inbound:



Then, added this security group to the instance, and it worked for me.