The water column detail in the SI image is more than just a lateral line to bottom on each side left and right ... (as you noted by being able to see bait balls in the dark water column area of the SI image) ...
You are correct the HB “folded paper” scenario is misleading ...
This diagram is a better representation of the SI image ...
Because Sonar measures distance - every target that is closer to the xducer than the value of the water depth will be painted in the dark water column area of the SI image ...regardless of which angle that target is from the xducer ...
The dark water column in the SI image should be interpreted separately from how the bottom detail is interpreted ...
The HB software engineers took the sections of the
bottom detail (left and right) and “shifted” those sections of bottom detail
outward in the SI image to make room in the SI image for the dark water column ...
Using the diagram below we can see how the sections of bottom detail have been shifted outward in the SI image to make room for the dark water column ...
Rickie