Scanning on their cloud storage is different from scanning on users devices. The latter is opening a door to mass surveillance that wasn't even there before. That is the problem, not the scanning itself.
If it hasn't been a problem that Google has been scanning your cloud data for the last decade, it didn't suddenly become a problem now.
There is no difference.
The feature what everyone is afraid of (scan all in my device), is super trivial to add generally. Company like Apple can push it to public in less like week regardless if this Child Safety came first.
This new feature is actually really hard to develop, because they try to create E2EE system with backdoor. And they want to lock themselves out of this backdoor to prevent misuse.