[Xeanort]

You are right that they cannot be added to History, but the code used here changes the back button functionality with

  $(window).on('popstate', function() {
    window.location.href = 'https://example.com';
  });

I just tested it and it works with different domain in latest Firefox.

[Ajedi32]

Fair point; popstate allows you to do pretty much anything when the history entry is for the current domain.

That's not really an issue for this particular attack though, which relies on the reverse scenario: the user remaining on the current domain when they expected to navigate back to the third party search engine.