However, in order to generate the server (and the client), a complex build system is being used. It is not just about generating the “WoW.exe” and “Server.exe” files. The build process takes data, models, maps, etc. created by Blizzard and also generates client and server specific files. The client only has the information it needs and the server only has the information that it needs.
This means that before re-launching vanilla realms, all of the data needed for the build processes has to be gathered in one place with the code. Not all of this information was under a version control system. In the end, whichever of these parts were lost at any point, they will have to be recreated: this is likely to take a lot of resources through a long development process.
I took it as both, sounds to me like code for server side realm / client side data generation in order for each side to talk to each other is missing. It's actually an incredibly vague description of what (I think) he's trying to say but that's probably for good reason and no doubt we'll hear more details during Blizzcon.