fingon I don't see any reason why Flarum wouldn't be able to handle that many users. Although, because flarum is still in beta, you may face issues other than slowness. Posible downtime because of bugs or simply the lack of features may be met. Use at your own risk in a production environment.
Personally, I believe you could use flarum in production if you know enough about it to fix issues quickly as the arise. It's important that you back up your database and assets regularly in case of data loss. i would also recommend using your composer.lock file properly as it should in a production environment. Let me know if you have any questions regarding this if you do wish to use flarum, regardless of the warning.
