It seems like he needed specific understanding of performance profile and writing your own is a good way to make that guarantee. Granted, language.js has some very interesting features that I've not seen in other PEG parsers so it's already pretty promising.

As far as the second edition of the language, I think they've got a good foundation with 1.0 on where the grammar gotchas will be (and specifically that the syntax was designed to avoid parser overhead since the days of ObjC). A powerful PEG parser will also make iterative extensions to this a lot easier (in my experience at least).