Filing certain parts down can give you a full auto weapon, but no ability to stop firing until all rounds are fired. From the video’s I’ve seen/heard, it was full auto, there wasn’t an irregular timing between the shots that would occur if someone was just pulling the trigger as fast as he could.
On an AR15/AR10 with stock trigger, yes grinding down a certain part allows or causes the hammer to follow the bolt carrier group, with firing pin extended as the round is being chambered.
While crude and perhaps effective, one never knows when one of the rounds is going to hang up a bit on the way into the chamber and fire out of battery- ie, with the bolt unlocked. When this occurs, espc in an AR platform, immediate and dynamic release of propellant gases render the weapon useless and often the user injured.
I suppose an idiot hell bent on something would do it though.