I believe part of my confusion was caused by the "caching behavior" of Xcode and the Simulator. Initially, I managed to sign in with a gmail account using the bundle ID, com.google.DaysUntilBirthday. After that none of my changes to Google's client ID or its URL scheme seems to matter. I'm still puzzled by this behavior because I think an invalid client ID should cause some kind of error.

At some point during my troubleshooting, I erased the simulator's content, restarted Xcode, I then observed the same behavior with both bundle IDs. I think this is a good tip to remember, i.e., resetting the tool/environment when some strange behavior/result is observed during testing.

My test project is available in github.

When I use an invalid bundle ID, the sign-in still works. I need to understand why that is the case.