fn f1() -> Result<(), E1> {...}
fn f2() -> Result<(), E2> {...}
fn f3() -> Result<(), E3> {...}
fn f4() -> Result<(), E4> {f1()?; f2()?;}
fn f5() -> Result<(), E5> {f1()?; f3()?;}
fn f6() -> Result<(), E6> {f4()?; f5()?;}
To my understanding, reverse engineering algorithms and interfaces is not a copyright violation since those cannot be copyrighted (i.e. fair use is not relevant). However, a WoW server also distributes e.g. quest texts, which most certainly are copyrightable, since the collective of all quests is comparable to a fantasy novel.
In backend terms (which isn't really relevant in court but helps illustrate the division), every WoW server is said to have a "core" that contains the gameplay logic (netcode, movement, hit rolls, object interactions, etc.) and a "world database" (item names and stats, NPC names and stats, quests, etc.). The core might be considered a collection of clean room reverse engineered algorithms, which aren't copyrightable. However, the world database is full of copyrighted material, and a server distributing that data to clients will violate Blizzard's copyrights. You could avoid this by deleting all of Blizzard's stuff from the world database and writing your own content, but it's not relevant here since Turtle WoW didn't do that.
Nonetheless, vbezhenar's point stands because there are open source server implementations that host both the core and the database on Github, see e.g. https://github.com/cmangos/mangos-tbc and https://github.com/cmangos/tbc-db.