They also say this:
3.3.1. Cloud chats
telegram is a cloud service. We store messages, photos, videos and documents from your cloud chats on our servers so that you can access your data from any of your devices anytime without having to rely on third-party backups. All data is stored heavily encrypted and the encryption keys in each case are stored in several other data centers in different jurisdictions. This way local engineers or physical intruders cannot get access to user data.
3.3.4. Public chats
in addition to private messages, telegram also supports public channels and public groups. All public chats are cloud chats (see section 3.3.1 above). Like everything on telegram, the data you post in public communities is encrypted, both in storage and in transit but everything you post in public will be accessible to everyone.