That is in theory.... In practice If you have a game that look pretty well on consoles you would have big challanges for mobile devices, and just reducing quality on art assets might not be enough.
Unreal iOS uses the same toolchain as the PC and console versions, including the same scripting system - that's the point of having a multi-targeting engine. Unless you modify the engine substantially you probably won't need to fork. In fact, if you use the UDK you can't modify the engine source anyway. In most cases you'd probably worry more about assets that work on multiple platforms rather than code.
But anyway, I think this exercise was to have the option of deploying to multiple targets, not every single target at once. The difference between the Flash, HTML5 and iOS versions of your game could be very small. For another game you could use the same toolchain to target PC, Mac, consoles, etc.
Well in the case of the game we were working on there was no choice other than fork our codebase. No considering the controllers (which is different enough from the consoles) the frame rate of our code base was awfully slow on IOS, so we had to rethink the whole project to target IOS properly. Actually I don't know any unreal game that sucessfuly have been launched on IOS and consoles with the same codebase, and after my experience with the engine I don't think you would like to do that in the sense that you always would like to exploit consoles capabilities. It's a tradeoff very hard to avoid.
