The article doesn't get into the math of how much of an equivalent bonus/penalty that Advantage/Disadvantage gives, and that is the actual complicated part of the mechanic.
On average the bonus or penalty equates to about +/- 3 on a roll. However, it can swing higher or lower depending on the difficulty of the check compared to the character's inherent modifiers. If a character can only pass by rolling a 20 (or can only fail by rolling a 1) after their other modifiers are accounted for, then the effective bonus/penalty is only +/- 1. Conversely, if the necessary roll is 11 or above to pass (or 10 or below to fail) then it is +/- 5.
The reason Advantage/Disadvantage feels so powerful is because D&D is balanced around players needing to roll somewhere between 7 and 14 to be successful in most of their actions, and that is the range at which Advantage and Disadvantage are most impactful.
This is exactly right. I was trying to explain the mechanic in terms of effective bonuses/penalties to show its effect more concretely.
Advantage doesn't actually confer a +5 when the needed dice roll is 11, bur statistically that's what it feels like.