Sion's Roar of the Slayer should be considered single-targeting.

Simply put, Sion's Roar of the Slayer takes on a different persona if it collides with a champion first. The armor reduction only applies when it hits a champion and affects no other target within the skillshot's range. I see no reason for this move to have AoE properties if it does not collide with a monster first.

A similar example is Karthus's Lay Waste (Q). This move is granted full spellvamp properties (in addition to extra damage) to striking a single target. An adverse example is Riot's decision to make Jhin's Deadly Flourish (W) a single targeting ability as long as it hits a champion in its path. These are both moves that have different entities upon meeting the right conditions and Sion's E should be classified as one of these special cases.

Of course if Sion strikes a monster first, the skill should retain its AoE properties as it transforms into a literal AoE skillhot.