Check out the 2nd to last line of that post. They make the same statement in doc...

reilly3000 on Oct 12, 2019 | parent | context | favorite | on: AWS Elasticsearch: a fundamentally-flawed offering

Check out the 2nd to last line of that post. They make the same statement in doc. Lots of services are getting VPC endpoints so traffic never has to hit the public web, but firehouse isn’t one of them(yet).