It sounds like this may not be happening on iOS. I have not found a way to access the Photos library, without the user being asked for explicit permission to do so.

But I also haven't really tried. I use Photos and the Camera in some of the apps I've written, and fully expect users to be asked. I ask for minimal permissions, as well.

If Meta is bypassing user permission, then that's a truly dire security breach, and Apple needs to bring down the banhammer fast.