The article talks about a "Traditional" front kick and issues associated with it. Some styles and instructors insist on the supporting leg to be facing in the direction of the kick. The acceleration of the kick is drawn from the sharp pelvic thrust. In kinesiological terms a posterior pelvic tilt.
The biggest issue with a front kick thrown into the target directly in front, is the hamstrings flexibility. More specifically lateral hamstring flexibility. Biceps Femoris (the two outside head of the hamstrings) are often tighter than the two medial heads. This makes the kick thrown directly in front more "flexibility challenging" than the one thrown on the side.
The supporting leg is a factor as well. Even if the torso is squired, but the supporting leg is turned out, the posterior pelvic tilt has more range. This is a compensatory range, when the pelvic has to move and compensate for the lack of length in the back of the thigh.
So to test how high you can thrown the kick without compensation, you can try this. Lay down on your back and lift one leg straight up. That is about the angle that you can throw the kick at.
It would be fair to mention arguments both ways. (For the range above or below the one shown in the test)
On one hand, with slight compensation from the pelvis, the kick can go higher. But those compensations can lead to injury, and reduction in speed and power.
On the other hand, a kick should not be thrown at it's maximum passive flexibility capacity, at full speed, without a target. This is because the hamstrings is used to decelerate the kick and can get injured, when forced to contract in mechanically disadvantageous range against a usually stronger quadriceps.