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.
It's definitely the case that the Firefox UI very much makes it impossible to move away from the current layout implementation at this point, and moving away would be a massive job. At the same time, the Firefox UI cannot move away from XUL/XBL until extensions no longer have free access to it, so this is definitely a first step that's relevant here.
As for using Servo for web content, it's worthwhile pointing out that not only is it the direction Mozilla is taking (with Project Quantum, and progressively moving components from Servo into Gecko), but it would also be a technical challenge because web extensions have access to extra APIs and have security checks disabled in others.