Bonus (2) of superior rune of sanctuary gives +20% frozen duration while the minor and major version gives +5% and +10% to boon duration.
Given that all other runes have a linear progression in the bonus as we go from minor to major to superior, I have a feeling that it should have been +15% to boon duration but for some reason it got messed up with Superior Rune of Svanir, which does give +20% to frozen duration for bonus (2). The confusion is probably because rune of the sanctuary is obtained by trading tokens from Honor of the Waves, where we fight Sons of Svanir.