Unfortunately, there was no way to make the _full_ extension work with only activeTab, as we have some functionality that is incompatible (one such example: if you open up a page you've previously saved and highlighted in Reader, we show your highlights overlaid on it seamlessly).
That being said, we do want to make another version of the extension that works for more privacy conscious folks, and will defs use activeTab for that! Just requires a little more engineering effort.
I totally understand the request though, privacy is super important. All extension data stays local on your device unless you actively press the extension button to save a page/content. You'd have to read our source code and network requests to confirm this right now, of course.
@tristanho - will you consider this?