One thing that's cool about this is that iOS must be using the slight movements of the phone in your hand to help triangulate the other phone. Ultra-wideband position finding relies on time-of-flight, so from a single reference point you only know the precise distance something is from you, and not its direction. To get around, my guess is iOS is using the accelerometers in the phone to measure slight changes a phones position, and then using multiple ultra-wideband distance measurements from those different positions to triangulate the other phone.