But aren’t room contents indexed and accessible from any client ?

With a Discord account, you can search in a Discord channel for any servers you have access to, as well as any of your private chats. This means you need to sign up with a commercial account and agree to lots of tracking and data selling, it means that information can be withheld by banning someone from a server, and that information could become very inaccessible relying on people screenshotting it and posting it on other places as has happened with information from Discord servers several times.