I wonder how their slicer is handling the infill angles. I suspect the bridge detection is only kicking in on the top layer, which ensures the infill angle is perpendicular to the gap. The bottom layer is going to follow a global angle and eventually run parallel to a hinge as the design gets complex (hence a later design failing and requiring thick hinges).

Painting infill angles would be tedious. I bet with concentric infill and some clever zero width cuts you could make all of the hinges perfectly perpendicular.