Yes socket.io announces to all clients, it does this by using redis pubsub. So if you have more than one nodebb instance it is advised to setup redis.
Can you connect to the database from the mongo cli client? If not, them you're using the wrong credentials.
Additionally, I recommend following the official NodeBB docs: https://docs.nodebb.org/installing/os/ubuntu/
Third party tutorials can be outdated or incorrect.