Ok, there are two separate plugins here, which is confusing.
discourse-telegram-notifications is an unofficial plugin, and not available on our managed discourse.org hosting. It is designed to be used to send users their personal notifications via telegram, and does allow basic responses. It does not work in telegram groups/channels. This is likely the video you saw.
discourse-chat-integration is an official plugin, which is available on our managed discourse.org hosting. It’s designed to be used to send new posts/topics on Discourse to a telegram group/channel. Replies are not supported.