Apparently the list of offending sites is hardcoded in the extension ( https://github.com/klinskyc/PTOAlert/blob/master/sites.json ) and the extension is not updated to reflect changes on the sites.

I suppose the only downside is the list of reformed sites using improved password security appears to rely on user verification and submission, so the list may not always be up to date - but it's certainly better than not knowing at all. Kudos to the extension maker.