Uh, no. It’s a lot more complicated than that.
Also, you seem to be unaware that private server code (MaNGOS) has been years in the making, is constantly evolving, and the servers that up to now have been considered accurate spent ages on research.
Blizzard started out with actual 1.12 backup code, and yep, they “modernized” it by getting it to work with the current World of Warcraft system. This was so they could use modern security features, put it on the launcher, and all that. The rest of the time spent has been in fact cleaning up old bugs and putting on “Blizzard polish”.
To be honest, they’ve spent less time on this project than the years spent on MaNGOS. Sure, they probably could have thrown a working version of Classic up six months ago and walked away. But it would have looked like a buggy private server and that’s not what they’re about here.