It takes an interesting approach to crawling where
    it renders the page using WebKit, and then exposes
    the "rendered DOM" -- so that your crawling code
    doesn't need to actually use JavaScript for
    information extraction.

It is an interesting approach. There's evidence that Google crawls the web that way, though I don't know if it's been confirmed by the company.

GoogleBot indexes content rendered by Javascript - even content delivered by an AJAX request. They've announced they are going to start penalizing sites that don't work well on mobile. I don't know the specifics of that (and they probably haven't shared them) but I do know that I've received automated email from Google Webmaster Tools and/or AdSense about one of my sites not working great on mobile: small UI elements grouped too closely together, content that's too wide, etc.