of course its lag but this causes 1 of 3 scenarios (like in the thread link said):
Scenario 1, Balanced Lag Compensation
In a properly working game where lag compensation is minimal or non existent, the player model and hitbox are in nearly identical positions. The Aim Assist sticks to the player model because that is where the hitbox is. You fire at the model and you are rewarded with hitmarkers and a kill and Aim Assist helps you along the way.
Scenario 2, Advantageous Lag Compensation
These are your "unstoppable" games. Your opponents seem terrible, oblivious, and slow to react. From a mechanical perspective, your opponents hitboxes are actually behind their player models. They come around a corner, you see their player model and begin firing, and their hitbox (which is where they actually are on their screen) essentially walks straight through your stream of bullets a quarter or a half second later. Since your cross hairs were on their player model, and their hitbox essentially walks through your crosshairs a short time after, the aim assist sticks to it and this often leads to your aim being pulled along slightly behind the model rewarding hit markers on thin air.
Scenario 3, Disadvantageous Lag Compensation
These are your games where you couldn't hit the broad side of a barn. Everyone is better than you. You get owned despite having the apparent "drop" on any given enemy. From a mechanical perspective, your opponents hitboxes are actually in front of their player models. They come around a corner and you begin firing, but their hitbox (where they actually are on their screen) has already passed and you are firing at their player model. Since the hitbox is already passed your crosshairs which are on the player model (assuming they were strafing across the front of you), you receive no help from the Aim Assist because in order for it to function, it must have come into contact with the hitbox at some point. Aim assist sticking to something that isn't there is highly counter intuitive! It pulls your aim away from what your supposed to be aiming at (the model), causing you to not only think you have to readjust your aim, but typically makes your overcompensate in the opposite direction because it takes additional stick pressure to overcome the nudge.