Let’s start with the obvious:
Of course 5 people with no experience in Vanilla WoW, who leveled up to level 15 and then entered RFC, would’ve been able to clear RFC. Vanilla RFC has zero mechanics. It’s literally just mobs with more hp. You could clear it with 5 dps if you tried.
I mean I used this example beforehand, but I played WoW for the first time when I was 12-14 years old, while clicking abilities and auto attack with my mouse, yet somehow my first character, a hunter, together with two real life friends who played the game just as much as I did and two random people we picked up from somewhere, were able to clear deadmines when we first went into it.
Second:
I believe people who started in vanilla at level 1 and leveled up to level 15 would most likely have more playtime in the game than a person who leveled from 1-30 right now, thus gained more experience in how the game functions.
If people who didn’t play WoW beforehand were unable to clear dungeons in vanilla, no dungeon would’ve ever been cleared, because, well you were unable to play WoW before WoW was released.
The first dungeon you entered was always a very simple dungeon with little to no mechanics. RFC for horde, maybe Wailing Cavern. Deadmines or Stockades for alliance.
You can gladly compare vanilla RFC, WC, Stockades and DM, or cataclysm RFC, WC, Stockades and DM with Atal’Dazar and check how many abilities the NPCs in the dungeons have. Then you’ll have an objective comparison between the dungeons and can come back to argue about how you didn’t beat RFC on your first try and thus Atal’Dazar is a good beginners dungeon.
Yeah that’s pretty much the issue. It makes sense from a lore point of view to give new players a “what happened last” summary, but the dungeons are simply not beginnerfriendly whatsoever.