Can you describe the build process? Building 100's of millions of lines of C++ in 20 minutes is quite a feat even today. How much parallelism and build distribution was used?
But the result was achieved mainly by lots of compacting source code into smaller TUs, mainly, e.g. by #include-ing dozens or hundreds of .cpp files into the one that was actually sent to the compiler, and other tricks to make the number of the things the compiler had to do much smaller than a naive ("compile everything as seen in the source tree") approach would require. The original build system was not optimized in that way and took much, much longer (well over an hour, as I heard it, but that was before my time). They implemented parallel and distributed builds after I left, and as I understand it that took the whole process down to "a few minutes."
Now, it's certainly debatable whether that level of effort should be required of a build system (I lean towards "no").