IIUC createdb has the good description [1] and it can be re-used everywhere.
As none of those programs "create" a database the wording "The default is to create a database" seems quite a poor choice to standardize on.
The wording you suggest replacing seems fine as-is.
ISTM the following is the only issue that probably needs to be addressed. Either by fixing the apparent bug in the code or documenting the correct behavior in the pgbench docs.
# ... but pgbench cannot $ pgbench -U postgres pgbench: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: FATAL: database "hayato" does not exist pgbench: error: could not create connection for setup
Your patch 0001 doesn't really seem to do either.
"connection to default database failed" is a strictly worse error message than the existing one used unconditionally.
There probably should still be the existing "else" block in the other hunk - we do default to OS user name - but to handle -U shouldn't we just add an "else if (username) ...block before it?
