This is 100% false, range finder existed in Classic. I used it on C'thun. It was simplistic, but it was there.
Example:
https://www.youtube.com/watch?v=fZlwnu8FQE0 - notice the green dot that turns to yellow/red as you get near players. You had to stay away so you won't chain the eye beam thing and gib the raid, it did double damage on each next jump.
As a matter of fact - Vanilla addons were pretty damn functional, addons you see now have a lot to do with years of evolution and discovery, I am pretty sure much can be recreated in Vanilla, but just like in that vid... what do you really need there? You got your boss mods addon, there was a damage and threat meter, you could get addon that changes raid/group ui, you could track buffs/debuffs.
You did not have WeakAuras, but I am sure that was more to do with a fact it wasn't made yet and not that it was impossible to make.
As a matter of fact API opened to addons was liberal than what it is nowadays, you could do shit like rotation bots, smart healing and so on with macros, because for example targeting people and casting spells was enabled for addons. They locked it away sometime after AQ and before Naxx, because it got out of hand.