Agreed, the snapshot does output a message in the logs but based on our conversations with AWS it was suggested that we use the SQL Command to determine the LSN. Sometimes depending on revision you won't get the logs and other times the log line is emitted twice based on the internal RDS consistency checks. Makes me long for GTIDs from MySQL MariaDB Galera[1]. They worked super well and we never looked back at my last company.
[1] https://mariadb.com/kb/en/using-mariadb-gtids-with-mariadb-g...