Sounds potentially useful, but question...

If you're trying to call 911, or you're trying to video an assault... and someone tries to stop you, such as by jostling you or your phone... what's the likelihood that this Private Lock will get in the way of that?

I believe Android has an SOS feature where if you click the power button five times in quick succession, it can take a photo and send a very scary-looking SMS message to a relative or some other trusted contact. I don't know if it lets you set it to call 911 or take a video though.

You can do both of those things from the lockscreen of any android device I've used in the last decade. The linked app just seems to engage the normal android lockscreen.

Is it really "Big Brother" when the model runs locally on your device and doesn't send any personal info to anybody?

Ideally no, not "Big Brother". But this is Google, and the burden of proof is on them to not collect data "to make our services better", etc.

Especially location data. If something like Find My Phone is activated, my immediate bias is to assume Google is taking advantage of this data collection to monitize my data without my consent.

It's not clear that any sensor data needs to be sent to Google for thois to work. I suspect this is all on device.

You're saying it works great, which makes me wonder how often your phone has been snatched away?

Google already has this data, because every app has access to sensors by default and GMS has always on location access anyway.

Even on graphene OS, location without GMS is almost unusable.

I expect it can work more efficiently as part of the OS rather than as an app.