Are you in any way affiliated with Final Draft or CeltX and/or is this an official Final Draft or CeltX app?

Nope. I'm a user of Final Draft and CeltX going way back, but I don't work for or with them. The app itself has some Terms & Conditions to read, but here's a little teaser...

Except as where explicitly stated otherwise, this app is not created, supported, affiliated, or endorsed by Final Draft, Inc, CeltX, Trelby, Fountain.io, Quote-Unquote Apps, Screenplain, Innovatics Inc., and/or any other app(s), corporation(s), people, licensees, or institution(s). Except dubscript.com, linked external sites are independent and may have changed or disappeared since the links were created.