When you say "locally" it usually means it uses the browser's API to navigate the inbox and perform whatever actions necessary. This extension instead requires pretty intrusive permissions, just like the alternative competitors (which are under fire for selling your data).
I appreciate that you state that "data doesn't leave your device", but the whole point of "local" extension is that I don't have to take your word for it.
I appreciate that you state that "data doesn't leave your device", but the whole point of "local" extension is that I don't have to take your word for it.