They have this reputation because, it turns out, not every engineer at the company needs to be able to rebuild all of Google from first principles in order for it to keep running. And even at Google scale there's not enough interesting greenfield work to do to, or de novo problems to solve, to keep all those "famous engineers" busy all the time. The same is true at most companies -- you need a mix of talents and skillsets, and you need to match up people to roles based on that. The Google approach -- of putting people into drudge work and paying them enough to hope they won't quit -- is a grossly inefficient use of talent and knowledge. Which is another reason I wouldn't want to work there!
I'm not arguing that Google doesn't have a huge scale, but I'm pretty sure that Microsoft, Facebook, Amazon and most likely the NSA also have unique systems and challenges.
And all of them (well maybe not the NSA not sure on that one) do the entire "algorithms quiz".
What distinction are you making between "expertise", "knowledge", and "outside experiences?" The outside experiences are the basis of the expertise and knowledge. If you understood, used, and contributed to other existing build systems, you are then in a good position to understand, use, and contribute to Google's in-house build system, because you have useful knowledge about the problem domain of building software. And so on.