This is not the case for any modern chat app. When you send a message to someone, it isn't delivered peer to peer straight to your recipient. The message goes from you to the service provider's server from where delivery is attempted whenever that message is processed. Your recipient might be offline, out of cell service, phone turned off, or etc and the central server takes care of that problem. Similarly when you log in to the chat service on the web, on your phone, on a new device etc, the messages need to be synced to you from somewhere, and that's again the service provider's central server. All messaging services do this. These days, some encrypt your messages, but not even remotely all.