That app is most likely a fork of OnionBrowser, an iOS app made by a Tor developer. That's the only 'official' app that you should be using, and it had a third party security audit.
Also note that it is underfunded once it became free on the App Store, so if you want to support its development you can check its page on Patreon.
Shame that some fork other people's hard work then sell it.
In addition, Onion Browser by Mike Tigas is open source. I know the Red Onion Browser looks more feature rich but is likely a watered down version and probably has some compromise within the code base.
There are quite a few unofficial apps abusing the Tor name. The only official mobile app is Tor Browser for Android (though Onion Browser for iOS has been mentioned by the Tor Project).
If you are using any other apps, you should probably stop using them and use the official app instead if you're using Android, or Onion Browser if you're using iOS.
> Is there a legitimate Tor web browser for iOS?
If by legitimate you mean an official browser by the Tor Project then no; though there is an unofficial Tor browser, Onion Browser.
Comprehensive anonymity is likely impossible on iOS. Onion Browser will route your traffic through Tor, but may leak DNS requests when loading certain content. It's open source though, so that's good. There have been fakes on the Apple Store in the past.
See the caveats here: https://mike.tig.as/onionbrowser/#caveats