That's not a very convincing proof. But you're right we don't know the spec and gear, I'm just assuming the world 3rd C'thun mage and private server mage understand how to spec and gear vs random vanilla mage with t2 shoulders. That said, I'm positive when I hit Ragnaros with frostbolt in BC he took extra damage from it so it would make sense to me.
If you're right it would also mean that 33 random frost mages in vanilla, when no one really knew anything, could sustain between them an average of 500 dps which from most actual vanilla dps meters I've seen seems unlikely. Most people were sitting around 200 dps or less in Blackwing Lair, that's how you get stuff like 30 minute Nefarian kills which weren't uncommon.
Also, I'm comparing two vanilla raids and one private server video.