First of all, why did you modify the post you quoted without pointing it out?
That is a good point though. I run full armor penetration marks so I never really considered if flat AD or scaling AD is better. HOWEVER, most people consider that AD marks are better for early game, while ArPen marks are better for late game, so its reasonable to assume they would use flat if they were going to use any AD.
Still, at lvl 18 its pretty easy to have over 200 AD, so the scaling AD marks still only give you an 11% damage increase, at most. Again, you have to decide for yourself based on your champion and play style.