Their plan was to scan stuff that would be sent to their hardware on your hardware so that they could encrypt it on their hardware without having the key themselves. And law enforcement wouldn't have had the "what about child pornography" -angle to force access to data, since everything would've been pre-scanned.
If no data would've been sent to their hardware, no scanning would've happened.
It wasn't a complex idea, but The Internet decided collectively to misunderstand it.
