There’s no mention of it being a strictly UK affair in the app description. I imagine 100s of people download it each day, fill in the form and then bail once they realize it’s UK only.
I've just checked the description in the Play Store and it says 'Open a full UK bank account'. Maybe that is new, but it is pretty clear.
That doesn't mean that it's closed to non-residents.
Don't all UK banks require a UK address?
But that's not something you'd expect a non-resident to know. Making account opening criteria transparent wouldn't hurt anything and would save everyone time.
I wouldn't expect a non-resident to know without doing research. But I would expect them to do that research because opening a bank account in another country because it is not a standard thing to do. I don't think it's the job of an app store page to explain this.