HTTPS Everywhere does not block all non-https requests since it would block a significant fraction of the Internet for users, hurting usability. Instead, it uses a ruleset to only modify requests for specific domains. You can search the list here: https://atlas.eff.org/index.html. Note that
uniquemachine.org is not in the list, so the HTTPS Everywhere extension does not do anything for this domain. You can ask the website owner to add themselves to the list here.
You can enable the EASE (Encrypt All Sites Eligible) mode in HTTPS Everywhere which supposedly blocks all unencrypted requests. To do this you must first right-click on the toolbar in Tor Browser, select "Customize", drag the HTTPS Everywhere icon to the toolbar, open a new tab, click on the HTTPS Everywhere icon on the toolbar, and enable "Encrypt All Sites Eligible" mode. Enabling this option may make you more fingerprintable due to non-default settings.
Firefox itself also now has an option to block all non-https requests, so this feature may come to Tor Browser in the future.