[Someone]

I haven't listened to that presentation or read the PDF, but from what I read about it I would bet that 'evolve' refers more to the time until the official release than to the time after that. Of course the language will evolve, but it will be slower and shouldn't do so in incompatible ways.

Until official release, though, Apple may make huge breaking changes (change keywords, tweak the grammar, maybe drop a feature or two. For example, do we really need trap on overflow for integers in what, in the end, is a compiled language?)