It is sometimes hard to imagine the scale of Google. If you can implement a task-switching mechanism with lower overhead and save 0.1% of CPU cycles across the fleet you can save many millions of dollars a year. So they have their own thread-switching mechanism which their binaries that can't run without. If you can improve process isolation in a way that allows you to pack 10% more tasks (by RAM or CPU) onto a single machine without affecting performance significantly you have just saved billions. So they have different isolation interfaces that Borg can't run without.

Both of these are completely feasible savings and make custom binaries and a custom kernel completely with it.