Communication safety in Messages is a different feature than CSAM detection in iCloud Photos. The two features are not the same and do not use the same technology. In addition:

Communication safety in Messages is only available for accounts set up as families in iCloud. Parent/guardian accounts must opt in to turn on the feature for their family group. Parental notifications can only be enabled by parents/guardians for child accounts age 12 or younger.

https://www.apple.com/child-safety/pdf/Expanded_Protections_...

Yeah, for now this is what is stated. It should be alarming how much they pivoted and changed with out any background statment. Something more is at play.

Thats the thing - with an unauditable black box like iOS you have to trust the vendor that it actually does what they say.

If they suddenly start to break you trust - who knows what they will push to that black box next time ? You have no way to tell or prevent that, other than leaving the whole platform altogether.