Legally, I'm pretty sure public domain is public domain (it's not just PD 'in the us'). You can do what you want with it without fear of copyright.
And it's good style guide, but the US government is a pretty big brand to compete for an identity with - it doesn't really seem like it would be a good idea to just adopt their style guide wholesale.
However, using this as a framework to build your own style guide sounds like a pretty decent plan.
Unfortunately, some other jurisdictions don't have a concept of "public domain" works. That's why the page indicates "... this project is in the public domain within the United States." in its license [1].
But you answered the main point of my question, so thanks!
However, using this as a framework to build your own style guide sounds like a pretty decent plan.