As someone who’s project was vendored into Fuchsia, I will say that there were a number of contributions that were made directly to the project to support its use in Fuchsia.
This means that while the overall project is used outside of Fuchsia, google is contributing upstream patches, to many of those projects specifically to support Fuchsia. At that point it’s not a clean separation of what’s “in house” vs. not.
Yes, but if you think about it, it doesn't make the argument weaker but arguably stronger - not only they use all those lines of code but also it seems that you can do it, ie. use low level (I'm guessing) 3rd party libraries and it will work for you; and they must be having high bar for quality, they operate on low level etc.
