They can see the image - why would they import a random image into their library from someone they don’t know?
This is a smart thing to disable, even outside this recent discussion of CSAM.
https://faq.whatsapp.com/android/how-to-stop-saving-whatsapp...
Edit: Though, to be fair, the specific hash-collision scenario would be that someone could send you something that doesn't look like CSAM and so you wouldn't reflexively delete it.
Personally I don’t really see the issue.
All you have to be is accused of CP for your life to be destroyed. It doesn't matter if you did it or not.
If the government wants to get you they don’t need this Apple scanning tech, or anything at all really