The law has ridiculous overreach, that's true. And we haven't seen what international enforcement of it looks like yet. But to deal with the facts as they are now, the law states that it applies if you have UK users, and that the personal liability for officers of the company can be up to £18M... The overreach continues because it also covers "harmful but not illegal" content.
The app publishing didn't exclude the UK, it probably should.
I don't agree with the law, there's no point in using emphasis to labour the point, the UK Govt advice on who it applies to is here https://www.gov.uk/government/publications/online-safety-act... and feel free to read it. As to enforcement, it's a new law so there has been none, hence no-one can speak to that, but you're right that extradition is the path and on that front if a similar law exists and agreements are in place, then for example most European countries would extradite. We don't know where the author is, but they posted on here in the morning of European time zones, and all I suggested is that they do their diligence on their personal risk as a result of laws that do claim to apply their service. No-one should dismiss the risk, the person should speak to a lawyer.
Your site has young trans people, sexual content, and would also be a target for grooming from chasers. The risks, for you, are very high.