Earlier quoted context omitted.
It only scans images in your iCloud Photo Library. Not paying for iCloud? No scanning. Not in your photo library? No scanning. And even then, only for known content, not new content.
So the attacker only needs to get access to your iCloud. Your iPhone will happily sync down photos uploaded elsewhere. You don't have to be paying for iCloud, either. There's a free tier, so I'd imagine almost all iPhones are using some tier of it. iCloud account break-ins aren't exactly rare. An accusation, even if false, could ruin an innocent person's life.
Or are you saying that malicious activity is only interesting if it was on an Apple device?