We can't integrate that into XenForo. It uses the viral GPL license, meaning that in order to integrate it, XenForo would de-facto become GPL licensed too.
It does have a JS snippet to be inserted in webpages. But unfortunately it's of no use unless you install Piwik on your own server. See the FAQ entry over here:I believe that comment reflected doing an integration of it (hence xenPiwik). If it's just a JS snippet, then that would be allowed.
How does Piwik work?