Yeah, but you can't do both, that's the point! If you can directly change the website content, the website itself runs in a sufficiently non-sandboxed environme

Yeah, but you can't do both, that's the point! If you can directly change the website content, the website itself runs in a sufficiently non-sandboxed environment to allow the data to be sent somewhere else. You can only get around this by making a more limited blocking process (which may not need to be as limited as chrome has gone with, but still, not being able to e.g. substitute in shims for ad networks will reduce the effectiveness of your adblocker. It certainly makes anti-ad-blocking have the significant advantage).