The private symbols in these builds could actually be very useful. The article alluded there were private symbols. So, even if only 1% of the overall windows code was leaked, if there were, say private symbols for the heap allocator of the kernel, for a practiced reverse engineer that is pure gold. Not as good as code, but a hell of a lot better than having to figure everything out and name functions and symbols themselves.
There are two levels of debugging symbols. One is released with every Windows build, for end-users reporting back stack traces and the like. These are of the second more granular level - private debugging symbols available to developers only.