That's no excuse. They could easily do that check at the point they find a clash. The whole point of this nonsense is that they're supposed to be detecting when another matching image is uploaded in the clear.
Either way, you can try uploading someone else's photo and, either way, at the point they're considering removing it they'll be able to see it. The difference is that if you hash it yourself on your own machine, they can't see it if nobody uploads it in the clear.
I cannot believe that this protocol is being seriously suggested as a way of protecting people's privacy.