The madlibs form has a "sign in as guest" link, while the original sign-in form does not. The article doesn't specify whether a guest sign-in is counted as a conversion or not.
Both forms have a 50/50 chance of having the guest link on them, and guests don't count as trial conversions (because I have several months of stats that suggests they give me money infrequently and they cause more support issues than all trialers and paying customers put together). The thrust of that A/B test is to see whether aggregate sales go down if the guests option is taken away. If they don't, I will drop it.