While that is true for most of vanilla, there is a few fights where you cannot really carry bad players.
4 horsemen, kel'thuzad Thaddius and C'thun are fights where the entire raid needs to both know what they are doing, and play your class well. 4 horsemen is the most forgiving fight out of those 4. The others are fights where one mistake can wipe a raid.
But about 4 horsemen.
This is a very unique fight in vanilla, and has enormous amount of coordination for your entire raid group, while the bosses have abilities that are very deadly if not handled correctly, thus making the learning part very hard.
1 missed taunt does not equal a wipe, but 2 can be very deadly. Not because that the tanks cant handle the extra debuffs, but because if you stay too long at one spot you might be too late taunting the next boss, and a few of these can screw up the rotation a lot so at one point a tank will have more debuffs than he can handle.
The reason it took so long to kill was because noone really figured out a rotation that worked well. Since every role has their own kind of rotation.
The guild Death and Taxes was the first guild to kill them, and they released (or it got leaked) their rotation just after the kill. And when the other guilds got their strategy, the boss was killed by several guilds almost imediatly. Causing death and taxes to lose the race for world first Kel'thuzad.
Which was so bugged that the world first was a complete RNG about who got the least gamebreaking bugs on a attempt.
After the rotation strategy got out, 4 horsemen was no longer a difficult fight at all.
The 4piece t3 helps a lot, but is by no means neccesary at all. But you just cant have too much bad luck in a short amount of time.
Many people also failed due to how the warrior taunts work.
Taunt transfers the threat to the taunter.
Mocking blow and challenging shout does not, it just make the mob focus the warrior for the duration, and once it is over it returns its focus on to the top agro target.
This was not really common knowledge, so many warriors failed by using those abilities, thus causing wipes.