Wouldn't checking the time (which should include TZ data) and comparing it with a database be adequate or can you not get local timezone data without permissions?

no, the timezones can be very very long north to south

You can get local timezone data without permissions in iOS.

But I think the OP's app is using the location data to ensure you walked a few meters to touch grass and not just have a patch of grass at your desk.

Yeah exactly, thanks!