The final design of this particular part is slightly different than the one showed in the videos. The blend surface you see above the 4-sided surface you drew over my model is no longer used as it was replaced by a large blend surface to enhance the looks.
There were two reasons why I opted to use a traditional sideways radius fillet rather than replacing it and a portion of the flat horizontal surface with a 4-sided patch as showed in your image:
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The CNC-milling of this particular part had to be done with a basic 3-axis CNC-milling machine while the part is being flipped to 90 degrees on the milling table (in comparison to its position on the chassis). The 4-sided surface you propose would produce a negative draft angle which is not good for use with a 3-axis CNC-machines. This is why the surface had to be kept vertical on the milling table, or sideways when viewed on the car. I explain that in more detail in the latest post here:
Aeromaster LMP - #33 by Rhino_Bulgaria -
From a pure design perspective, the sideways fillet is a great way to force the light reflections follow the same direction. An alternative rotated 4-sided patch could never deliver perfectly straight, sideways light reflections. This interior panel is designed to be produced either from carbon-fobre composite with a highly reflective coating or a matte fibreglass. The reflective coating easily shows the advantage of using a proper straight fillet over a freeform patch. But the main reason is point #1.
