Expect "Why I migrated from Go to Hack" articles soon enough.
Either way, the name is very fitting. I have no use for this, but good for Facebook that they've managed to (at least to some extent) evade some of the many PHP pitfalls.
Historically XHP was developed before Hack. It's available as an add-on for PHP but never became a formal part of the PHP language. When Hack was being designed it needed to work well with XHP (as XHP is used a fair amount with Facebook's code base), so over time it made more sense to start thinking of XHP as being one the language features that Hack offers that is not available in stock PHP.
Why would you expect that? Obviously the bulk of people migrating to hack will be coming from PHP.