How many Googlers are using Linux for their everyday work? I assumed MacOS had as strong a hold on Google engineers as those anywhere else. I personally don't like using MacOS and would be thrilled to see more developers using open source systems.

Pretty much all software engineers have Linux on their main desktop computer. On their laptops it's mostly MacOS or ChromeOS.

I've been very interested in exploring Chrome OS as a development machine even if the core system runs on a desktop like you said. Is ChromeOS good enough for this already? What about develop tools like editors/IDEs, etc ?

In my experience, Chrome OS is absolutely usable as a development machine for linux environments with crouton[1]. Crouton basically installs Ubuntu within Chrome OS, and can include GUIs and tons of software packages. I've done nearly the entirety of my WGU IT degree using Chrome OS, and if I get a software development degree, I'll use it there too. If Windows interoperability or virtualization is required, then you might look elsewhere, because virtualization is tough to set up in the chroot environment and would run sluggishly anyway if you use an ARM-based laptop like I do.

[1] https://github.com/dnschneid/crouton