Lets draw the line somewhere the other side of applications that advertise themselves as having that function?

it’s basically a cache with a fancy API so you can incrementally update data structure instead of having to constantly write the whole thing you are trying to cache.

If you mean “it doesn’t have every feature of Postgres” and behaves a little differently then sure but it absolutely is a database.

Watch is a poor man transaction and stored procedure does not work when you need to read value interactively from somewhere else (user/filesystem/another database) while inside a transaction.

I don't disagree with you about WATCH being less powerful than PG transactions. SQLite data types are less powerful than PG types but it's still a database.

All of this to avoid spinning up an SQS queue?

I also question whether you really bought a horse in this analogy.