I don't know about the USA versions of these houses but in Australia most of them are cavity brick walls which are at least 10 inches (250mm) thick and this would not require any false backing. The depth of the reveal as depicted would be correct as such a window is mounted on the exterior brickwork. This mean with the depth shown the shelve would approx 8-10 inches deep.

Thank you for your interest in our project. On this project the window seat is set within a bay window. If it were not, one option would be do have a false back as you suggest.