I did some research and found this.
On August 26th 2006, a bug was discovered by Blizzard that made entering the C’thun’s room significantly harder than intended, as explained in this blue post from Tigole (Jeff Kaplan):
We’re in the process of investigating C’Thun’s behavior in Patch 1.12. Unfortunately, we won’t be able to fully fix the encounter without a new client build. We’re hoping to get a Patch 1.12.1 together in the next few weeks that will fix the problem.
In the meantime, we’re working on a Hotfix that will temporarily give players more time to enter his room without suffering extreme consequences. We hope to have this hotfix up soon.
Apologies.
(I can’t include links in my post but if you just google the quote you will find the blue post, it is archived on mmo-champion).
As explained in the same blue post, they instantly applied a fix that reduced the “range” of the beam when players entered the room. This is the version we have now in classic.
A couple of days after, on patch 1.12.1 (September 27th 2006), they implemented a “clean” fix of the original bug, and reverted the hotfix, making C’Thun work properly again (beam goes out as soon as someone peaks at the entrance).
Again, it is explained in the blue post:
A hotfix has been applied to all realms which should make entering C’Thun’s room more feasible.
In 1.12.1 a cleaner, better fix will be applied. But for the meantime, the encounter should be doable.
You can also see the few days/weeks that the hotfix was live (before patch 1.12.1) in action in some videos after August 26th 2006 and before September 27th, for example here:
TL;DR: It is consistent with the reference client…but the reference client is a snapshot of a hotfix that was patched in a matter of weeks, and is not a reflection of how the fight is supposed to happen in vanilla. This needs to be fixed ASAP.