If you have 10000+ machines, then you already have the infrastructure to building deb packages, and experience in running apt/dpkg based system, then the jump to Debian is much smaller than jumping to Manjaro/Arch.

There's a huge difference in you switching your single desktop to a different distribution and switching an organisation the size of Google. Going with Debian brings then closer to "upstream", while leaving them with a base installation that isn't much different.