clarkwinkelmann you would have to include code for each and every of these extensions in your native app as well
PHP code into native mobile application? Seriously?
Flarum has an API. Javascript client communicates with backend via the API. It was not added as an afterthought as was done many other forums, but Flarum was originally designed this way from the ground up.
Nothing prevents native mobile application written in Java (or Swift, or Objective C) to use the same API. So, it is doable, not exceptionally easy, but definitely doable.
I think default web interface is quite good on mobile as well. So there is no particular urge for native mobile client at the moment.