I remember well those videos , with Peyton Manning etc on the O line , and yes they played ok as lineman , in the sense that they didn't get completely mashed every play as you would realistically expect , NOT as well as good lineman , that's an exaggeration
The reasons given , truthfully , at the time , were that the game just isnt/ wasn't designed to deal with such stupidity, it works based on dice- rolls and probabilities
In reality , on the average snap , the probability of any offensive lineman being destroyed is very low, and this is programmed into the game calculations, thus a low rated OL will get beaten more than a higher rated equivalent , but the formulae prevent ridiculous numbers of losses
Essentially any OL rated below a certain threshold thus would 'play' at a certain predetermined level , that satisfies the big picture, overall success rate
Its akin to asking a wine taster to grade wines using a scale if 0 - 10 , using the whole scale , then giving him a glass of water to grade and expecting a grade below zero
Edit: reading my answer made it sound like the system was working perfectly , which obviously was far from the case , but it wasn't as bad as those videos abusing it would have you believe