There's nested sandboxes for browsers in mobile environments. There's the inner layer which the web content is running in, but then the browser itself is sandboxed so it can't do things like access OS APIs it doesn't have permission for, install apps that run in the background, etc. This is why the iOS example needed 3 exploits chained. The fact that a similar example worked on Android, which also has app sandboxing, implies there should be an exploit chain but we've only been told of the first.