So what are they saying here?
We are talking about additive packages sold by additive companies, not finished oils here. They aren't saying that Group II base stocks are made to be used as additive bases to be used by the additive companies for their additive packages. They mean that Group II is an "optimal"
compromise as a base oil in a finished oil ("blend") for solubility and oxidation stability.
As the article explains, while you go from Group I to Group IV, the solubility gradually decreases and the oxidation stability gradually increases.
When you mix the additive base with the actual base oil, it will be diluted substantially and the solubility will decrease substantially. For this reason you want Group I for the additive base so that you start from the maximum solubility and even when it's diluted by the actual base oil, you have sufficient solubility.