Surely this developer meant that the leaked code had many dependencies on libraries that were proprietary to M$. That would make it difficult to build but this scenario is likely unique to partial code leaks at large companies.

Even a complete code leak can be near unbuildable without knowledge of how to set up a build environment. A number of years back a product i work on was so difficult to build it was a two day effort to set up a new developer workstation with the help of the existing team. Someone with just a full code dump would have taken weeks.

I read (old new thing probably -- a long time ago) that windows had circular dependencies until relatively recently. A large-ish team spent half a decade untangling it. Good luck building 40mm lines of source for a 25 year old project without detailed instructions...