I think they go into some of the reasoning in this post: Itemization in WoW Classic.
The main reason this wasn’t done appears to have been because the knowledge of the game can’t be unlearned and so the experience can’t be 100% matched. It’s inevitable that people would be much better prepared and clear content much more easily.
They do specifically mention that because of these reasons they chose to simply use the 1.12 patch as system reference. Regarding the 1.12 client I did find it very authentic.
What I’m mostly referring to, in my post, is the amounts of changes and bugs that have since taken place effectively dismantling that 1.12 client.