Does Ruin Runner get affected by Ahri legend ability?

Yes, Ruin Runner is affected by the Ahri legend ability.

According to RiftJudge FAQ #6834, the Ahri legend ability does not target because it automatically applies to all attacking units without requiring the controller to make a choice. Because the ability does not "choose" or "target" the unit, it bypasses the protection granted by Ruin Runner's ability ("I can't be chosen by enemy spells and abilities").