In that other thread there was a link to a YouTube video which showed an easier way. But for some reason the YouTube video is gone.
Basically consider the following scenario where the higher Walls cast shadows to the Lower Walls. This is how it looks in 3D Axonometry.
You want to create a section line between the two walls facing the lower walls. This way the higher walls will be behind the section line and their cast shadows are not calculated.
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