Thanks for your clarification. With the utmost respect (really), I'd personally only 'trust' the signify to show that I obtained the binary that is in the repository. After all, with the same xz example, that could still have been in our repo (if someone in good faith had compiled and submitted it).
A more 'glaring' bad extension would hopefully have more eyeballs (Not only the person that somewhat skimmed what was submitted, but also the other users (by usage) of the extension).
It would only guarantee that whatever is currently in the repo, is what I got, be it good, or bad.
Who knows, a fair poll could shed some light on what 'the masses' think about the subject. You may be very right that they would mis-perceive it's purpose (what it does, and doesn't add/do).