Exactly: Google detected they were hacked by the Chinese government a decade ago ("Operation Aurora" affecting a lot of other companies including Adobe, Juniper, Rackspace, Yahoo, Symantec, Northrop Grumman, Morgan Stanley, Dow Chemical).

Google stopped allowing Windows to be used within the organisation in 2010 as a response.

I think any other organisation that continued to use Windows values features more than they value security (and I believe that is still the case: you can't secure Windows, Office or Microsoft browsers against state level actors).

Source? Looked online and couldn't find anything apart from one or two untrustworthy mentions on quora.

I doubt anyone from Google is going to publicly comment on their internal security policies, but AFAIK this is a known fact for many years. I'm sure there's plenty of people who work for Google who can comment anonymously.