Your comment made me dig in a little more. I was wrong, it is only fetching the current tab, although it wouldn't need more permissions to see all the tabs.

In popup.js[1]:

    chrome.tabs.query({active:true,currentWindow:true}, function(tabs){ ... })

These `active` and `currentWindow` parameters to query() [2] restrict the results to the current tab. If I remove those parameters and run in DevTools, I seem to get a full tab listing.

[1]: https://github.com/pinoceniccola/what-hn-says-webext/blob/ma...

[2]: https://developer.chrome.com/extensions/tabs#method-query