"AppEngine doesn't support Go" Go itself is still pretty experimental. Changes are still made and with preparation for Go 1 they are making many backwards incompatible changes. They plan to remove the experimental tag on AppEngine the same time Go 1 is released. Until go itself becomes stable, they can hardly call the AppEngine version stable.
"no, without windows Go SDK it is not supported" Considering that the compiler they use for go (gc) is not supported on windows, that is not much of a surprise.
"Android doesn't support native full featured Go apps, although in theory it seems like a perfect match." This does not seem like a very good match at all. First, android GUI is programmed in java, it makes very little sense to try and make some sort of bridge between go and the java gui. Also, go at this stage is much better for servers than client facing applications. You know what would be an awesome fit? Go on AppEngine...
"Given that lack of support, I'm not even sure it is as widely used inside Google, or the story about inside usage is just a marketing." So far you have given evidence that Go is not being supported for people who use Windows and for programming phones. That means Google does not use it in house? The fact that AppEngine supports Go is an indication of the opposite. They probably wanted Go on AppEngine so they could use it themselves.