Tested it out. Using spreadsheet terms, it seems that when you want to select A1 to A2 it will scroll unless you have already started a horizontal select. The whole workflow is actually really nice and smooth. I personally like it. I would recommend others to try it out.
As with all of these things it's not obviously discoverable but once you know how the interaction works it's really useful. Seems like one of these features that you'll forever find yourself wishing for in every other app where you had to select a load of things.
We're going to introduce a short tutorial that makes users aware of the interaction when they first open the camera so that should make it more discoverable. Another thing that isn't obvious yet is you can deselect all the images by shaking the device! We'll make that obvious in the tutorial too.