It's the opposite. That is a link to the section of the Mandrill docs, not the Mailgun docs. The view_content_link option fixes the security problem. (In theory, anyway).
edit: worth noting that there are obviously other ways a hacked Mandrill/Mailchimp account could be abused. This just shuts down one of the major abuses you could perform.
Those are two entirely separate companies (unlike Mandrill and Mailchimp which is the same company.)