I think that's wrong. That it's bunkum. Golfers all try to keep a straight left arm with no bending of the elbow and to keep the same spine angle during the backswing and downswing. That straight left arm is your yardstick for the swing. Your left arm doesn't grow shorter or longer.
As long as your left arm is straight throughout the backswing, it doesn't matter if you have a flat swing or an upright one, or where your right arm is. The arc is the same. Except...
Except that the angle of your left arm with respect to your shoulders and the extent of the shoulder turn does affect the length of your swing arc. If you turn your shoulders to the maximum and keep your straight left arm in line with your shoulders, you have the longest arc possible.
Notice this means a flat swing.
All this assumes that your swing arc is on or near one plane. Now of you do hold your right arm high on the backswing and then drop down into a flat plane for the downswing -- which is actually what happens in most good swings -- then you have elongated the swing, but only because of that drop into another swing plane.
Think about it. What do you think?