When we want Revit to provide more contours than the source file provides it can but that's where we start getting some distortion of the contour lines we see. The rest of the video shows how to manipulate the contour lines with additional points to help Revit sort it all out.
Keep in mind that Revit creates triangles between points. You can see these if you use Visibility/Graphics to turn them on.
This is what the triangulation looks like before adding an extra point to help Revit draw the contour more accurately.
This is what the triangulation looks like after adding the extra point.
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