Earlier quoted context omitted.
Well, XUL-based extensions are not scraped yet. They will, at some point in 2017, but we're not there yet. The reason to scrape them is simple: the total API surface of XUL-based extensions is pretty much all of the internal APIs of Firefox, which means that any change anywhere in the code of Firefox breaks some extension accidentally. That's a compatibility burden that Mozilla could afford when the only competitor w…
And, related to this, it makes it easier to move to Servo's layout code in the future, as it avoids Servo having to implement XUL and XBL.
So for the foreseeable future, servo is not an alternative to replace XUL/XBL anyway. mozilla said that themselves.
Using servo to render the actual web pages inside the browser, that's another matter (once servo is considered stable enough, of course). To the add-on developer, this should not really make any difference anyway, XUL add-on, SDK add-on or WebExtension alike, as most add-ons for the most part will use the regular DOM APIs which servo has to provide anyway to interact with web stuff.