... What mechanism is used to perform incremental builds? It is referenced in the article and unexplained from what I can tell. What enables "quickly" for you to determine what needs to be built? Make simply compares timestamps I believe.

... What mechanisms/tricks speed up builds? Or is it just the "simplicity" that is the BFD?

Edit: It sure would have been helpful if somewhere was stated: "the build system of Chrome":-)

That statement indicates a huge difference between some rando build system on someone's personal project, and a build system on operating-system-size codebase that is over a decade old.