We know that Hydaelyn has tempered before. The cave paintings portraying her and Zodiark had smaller figures underneath them, sporting matching colors. We could get obtuse about it, and say that merely represented their factions, but I doubt that the Hydaelyn faction figured out how to summon a primal that doesn't temper on summoning, using the same summoning technique as the Convocation.

A line of thought I had, after Y'shtola and Krile aired their theory

At the end of 2.0 we see a flash of the crystals that represent the Source and the Shards. At first they are blue and bright, but then there is a flash, and they all turn purple and dark. Every other time we've seen them, they're portrayed in the blue. Looking back at that scene now, it seems to me as though the reverse could also be true. That is to say, we've never heard Zodiark talk to anyone. We assume this is because he is Sundered. and now with SHB, we also assume this is because he could be mindless from missing his heart. However, could it be that Hydaelyn can't take the time to talk to us, because she has to work actively to keep him mute/asleep? I for one, want to know, why is it that Hydaelyn can talk to everyone along all of the Shards and Source, but Zodiark never could.