The addon is entirely related to existing coverage of websites because all it does is replace http urls by https ones if they exists. It doesn't magically add https to sites that don't support it.
All those people who downvoted me don't understand neither HTTPS nor how this addon works. The addon is only at value when you request an HTTP url for a website that doesn't automatically redirect you to the HTTPS url. It's almost useless since the vast majority of websites automatically redirect you.
I thought the extension sends you directly to the https version, that is similar to how HSTS works, and securitywise alot better that http redirects.
Of course it doesn't magically add https. I never said it did. As you said it enforces use of https instead of http. That is what I said was necessary. The three reasons that is necessary were what I listed.
It doesn't yet, but theoretically it could redirect http URLs to archive.org/archive.is, which have https.