Scooping comes up in in this answer:
Move your hand in parallel with the strings (i.e. don't scoop)
What exactly does that mean? I've only seen it in reference to tone.
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As mentioned in my comment, it would be best to ask the user who answered that other question what he exactly meant. Here I can tell you what I think he meant, and also what my take on this issue is.
First of all I think that "scooping" is not at all a standard term when it comes to right-hand guitar technique. What I think is meant by it in the context of the question is the movement of the pick on a circle that is perpendicular to the plane of the strings (which is of course only an approximate plane, depending on the fretboard radius and the setting of the bridge). This movement is caused by a curling movement of the wrist (like playing badminton).
Instead, what (I think) should be done when playing fast downstrokes (this is what the original question is about) is a small up-down movement of the pick by moving it in the plane of the strings. This movement will usually also follow (a very small part of) a circle, but this time the circle lies in the plane of the strings. The movement won't normally be an exact straight line because usually palm muting is required and your palm is fixed to the low strings. So I think that a large portion of the movement should come from your wrist and not so much from your forearm, it's just that the wrist moves differently: it moves up and down and doesn't turn.
Particularly when using only upstrokes with tones on the same string, at least I get a more pronounced scooping motion when the pick moves towards the string. You still strike perpendicular to the string. I believe what is described is whether or not this motion is parallel to the body of the guitar.
If you try striking a a chord using up- and downstrokes, and you limit yourself to only move your wrist, not your lower arm, I think you will see this scooping motion. (Don't anchor the palm on the bridge, since then it won't be as noticeable).
In the answer you linked, the reference to scooping could have meant to keep your pick close to the strings as you move up and down the neck as opposed to allowing your picking hand to "scoop" (drop) below the neck. Obviously the only person who can say for sure is the person who posted said linked answer.
This advice applies to playing fast picking runs where you don't want any wasted motion slowing you down. So you don't want your pick to stray too far away from the strings you will be picking.
It does not refer to the picking motion itself (which would be perpendicular to the strings) but to your hand position as you move to different places on your fretboard.
To use an analogy - if you are playing a fast run on the 2nd and 3rd strings - pretend like those are the rails of a railroad track and your picking hand is the train. Don't let the train get off the track as it moves back and forth to play higher or lower notes on the fretboard.