Apple could publish a builder binary with which you could verify that your submitted code gets compiled+linked+thinned into the same thing you get from the App

Apple could publish a builder binary with which you could verify that your submitted code gets compiled+linked+thinned into the same thing you get from the App Store (minus the signature.) They haven't — which is bad — but they could, and that would be a guarantee. Same as you get with reproducible binary packages in Debian.