The additives are in liquid form. They are disolved into a oil that is known as the carrier oil. This carrier oil is then blended into the base oil.
IIRC, there used to be a problem with getting some additives to disolve into synthetic oil, so a conventional oil was used for carrier oil, so this statement WAS correct. With the additive package usually being around 15% of the oil, synthetic oils were partially conventional.
From what I understand, manufacturers have overcome the problem with additives and synthetic oils, making the carrier oil comment not currently true.