You are using the security argument again. It is not used in an adverserial context. You are correct, that this is not secure. And messages can be tampered with

You are using the security argument again. It is not used in an adverserial context. You are correct, that this is not secure. And messages can be tampered with. But this is not the application. The threat model of the application is that everything happens in a private context without adversaries. The communication is end-to-end encrypted and every participant of the chat has total control and is allowed to change everything in the chat, even messages from other users. So there is no point in protecting again adversaries that have access have to the secure channel, because they are already allowed to do anything, even if the hash were cryptographically secure, they are allowed to change everything. The integrity is only for synchronization, so that every participant can easily verify the state of the event history up to a specific point.