I find discussions around this kind of stuff frustrating because often times what's actually happening always gets muddled by the hysteria, even here on Hacker News.
From the link: "Before an image is stored in iCloud Photos..."
This leads me to believe that only data that is going to get uploaded to their servers is going to be scanned. If anyone has a different interpretation or thinks I'm wrong, feel free to reply.
It's my understanding that all cloud services do this type of scanning, when they are technically able to.
But all data on iCloud is encrypted by default, so Apple can't scan for this kind of material once it is on their servers. Doing it on device before it gets encrypted and uploaded is the only place they even could do a scan like this.
Additionally, they make it clear in the article that there has to be more than one hit (they don't say the actual number) which would mitigate risk of hash collision false positives.
If this type of scanning makes you uncomfortable, you can just not use their cloud services.
I do agree that this is still not a good direction to go, even with all the precautions they've taken. But I had to do some digging to figure out what was actually going on, the comments/commentary made it seem like Apple is now routinely scanning all your photos/videos if you have an iPhone.
Once the code is there to do local scanning, it might make it easier for a zero day exploit to do phone scanning and grab data it might not otherwise have access to or for governments to force Apple to conduct scans of content on a phone when they ask.