Do you use this?
Some people use it. Some people don’t. It’s not a requirement to use it.
I think they create the community of infinite flight with this app
Not necessarily. Discourse is its own separate service/website.
The app is just a “convenience” factor for users to access it easily rather than logging on to the website every time from a computer.
That’s just DiscourseHub, it allows you to get notifications for the forum to you device. And on phones you can can view the forum with it.