The following makes an explicit reference to the simple query protocol being the only protocol allowed in walsender mode. It is my understanding this is true for logical replication as well ??
51.3. Streaming Replication Protocol
To initiate streaming replication, the frontend sends the replication parameter in the startup message. A Boolean value of true tells the backend to go into walsender mode, wherein a small set of replication commands can be issued instead of SQL statements. Only the simple query protocol can be used in walsender mode. Replication commands are logged in the server log when log_replication_commands is enabled. Passing database as the value instructs walsender to connect to the database specified in the dbname parameter, which will allow the connection to be used for logical replication from that database.
