You're right, I think. The language that's been at the centre of Mathematica for 25 years hasn't had a name of its own (it's just been "the language used in Mathematica"), but now it has its own name - the Wolfram Language. The Pi has an early pre-release version of version 10, whereas Mathematica is currently on version 9, and there are a number of additional features which will presumably be included in the release of Mathematica version 10.

Leaving aside the re-branding issues, it makes some sense for the language to have a separate name, because Wolfram intends for it to be used in web browsers and other places where there is no surrounding development environment (the Mathematica notebook and 'front end').