out of interest are Google employees still on their own version of Debian? I remember them switching from Ubuntu a while ago.

Yup

That's a very good sign!

Do they mostly use Chrome (as opposed to something based on Chromium)?

Yet at Google IO is almost only macs at the sessions.

I've never been to Google IO, but I'd imagine that you see more Macs for presentations/sessions because they just work on the go. Working at a big tech company in the past, I had a beefy Linux workstation and a Thinkpad that ran Windows that I would only ever use for giving presentations/teaching classes, or while travelling for work.

When my friend used to work there, he was issued a macbook pro which was used mainly to ssh into his Linux workstation from a Chrome tab.

I hate to be rude, but IO doesn't exactly bring to mind the most clever or dedicated programmers.

You are only being rude regarding some of the key people working on Android, ChromeOS, Dart, Go and Chrome.

It is a matter of how their lack of cleverness and dedication might affect some of the products you might be using right now.