An unrelated HTTP session cannot set a cookie for another domain (unless it's a subdomain in which you have the more serious issue of session theft or session fixation). The solution to both of these problems is HSTS with the includeSubDomains option.
I've just woken up and I haven't tested it, but off the cuff I believe HSTS will prevent the browser from trusting a plaintext HTTP response at all. So you cannot force a cookie if I understand the blog post correctly. You'd have to create a cookie inside of a verifiable HTTPS connection, which if you can do you've already executed a much worse attack.
