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.
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.