Good point I didn't look at this and didn't say it. The target also says 13.0, like the project. So this appears not to be the cause..
gerdc
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<deleted, I was wrong>
Yes, I already set both to 13.0: the Project and the target (before submitting the app). It appears Xcode doesn't let me test with a simulator prior to iOS 13 and I have no physical device prior to iOS 13. Explicitly stating the required iOS version in the App description is a good idea, thanks.
Worked for me. Why ist this answer at the bottom of this page?