The scanning only touches photos which are stored on iCloud. Yes it does it on your phone but the reality is the same on iCloud or Google Photos. Both Apple and Google can push out changes live to every phone without warning, so what they may do in the future doesn't really matter. Only what they claim to be doing now. This is actually a more privacy friendly approach because it would be compatible with encrypted cloud storage.
The scanning only touches photos which are stored on iCloud. Yes it does it on your phone but the reality is the same on iCloud or Google Photos. Both Apple and Google can push out changes live to every phone without warning, so what they may do in the future doesn't really matter. Only what they claim to be doing now. This is actually a more privacy friendly approach because it would be compatible with encrypted cloud storage.