If you read the article, you'd understand that among ALL the issues, this is not one:

- Photos scanning in Messages is on-device only (no reporting to govt.) and doesn't turn on unless you're an adult who turns it on for a minor via Family Sharing controls. - iCloud Photos scanning doesn't take effect unless you save the photo and it's already in a database of flagged photos. So in your scenario, you'd have to save the photo received from the unknown number to get flagged.