Read the GP comment. This is talking about walking up to any random phone and having your own environment synced to it seamlessly. For that to work, the phone would have to know it’s you with basically perfect precision. So 1 in a million is not nearly good enough for that use case.

But you can only enter a pin on a phone you have physically with you. With this proposed auto-sync, every new phone is potentially a gateway to anyone else's phone.

Not that it's much better, but I thought iOS now required 6 digits going forward?