Onion Browser is an independent project created by Mike Tigas[0] a member of Tor Project team[1].

[0] - https://github.com/OnionBrowser

[1] - https://www.torproject.org/about/people/

I checked and it looks like Mike Tigas is a Tor Project core member, he is listed here as author of iOS Onion Browser: https://www.torproject.org/about/people/#core