I think the distinction here between "handing over your data" and "letting a model train on the data on your device" may be more subtle than you might think. There is still no guarantee of privacy - it is trivial to construct objective functions which probe data from your device.
I just skimmed their secure aggregation paper (linked in the post), and while I'm no expert, I believe they can actually guarantee privacy. At least for the strong version they describe (there's also a weak one which requires trust in the server).
