The loot system wasnt changed until mop. The changes made helped implement personal loot rolls, and some undocumented loot changes were reverted at that time (im assuming because players were getting double loot - boss spawn AND personal loot rolls) These changes set the way for personal first time kill only SoO Heirlooms.
Mobs roll their loot at spawn, the addon pinged the server to see if an item id existed - it only worked in an instance as the ping went only into that instance. It doesnt work in the world because those items may already exist so the ping picks up on that. Technically you could do the same with any drops in instances today, provided no copies of that item exist within that instance (eg bindings or mount)
All mobs in that instance would have rolled pre determined loot drops on spawning the instance id, so the boe drop was always there, and had the player waited, everyone would have had a chance.
If you look at past posts there is existance of this - a guild killed a boss, seen the loot but they all got dc’d. Upon logging back in they had to kill the boss again, and had exactly the same loot table. If it rolled upon boss death, the tables would have been different, due to the nature of RNG.