Low Point Reference to the ground is because it is where it is located, at least the relationship for up and down.
Full extension of the primary assembly defines the longest radius of the golf stroke from the left shoulder.
Left shoulder relationship to the low point ??
Your question initially was regarding the impact point to low point and thier respective plane lines.
The amount of down and out is to a large degree defined by the incline plane angle.
The reason for the test of the two balls was to demonstrate the 3 dimensional stroke, to show that the impact point is relative to ball position when the swing center remains constant. The converse to this would be to change the incline plane angle, ball position the same and note the change both in height relative to the ground and position to the target line of the Low Point.
Low Point in essence is the point at which the golf stroke changes from the down stroke to the forward stroke.
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