The Sentry Slack app can post alert notifications to a channel without being a full member of it. However, @mentioning an app requires it to be explicitly added as a member.
To enable @mentions, open the channel and invite the app directly: type /invite @Sentry in the message box and send it. Once added as a member, you can @mention Sentry to interact with it in that channel.
Why can't I @mention the Sentry Slack app in a channel where it's already posting alerts?
Learn about how to use @Sentry in a Slack channel Sentry is already connected to.