The problem is that the Resonance is basically the Echo. And yet Zenos and Fordola aren't connected to Hydaelyn at all. Arenvald never mentions her either. And there's a whole group of people with the Echo in 1.0 and early 2.0 who have no idea what the Echo is. Just that it allows them to do weird stuff the people around them find odd and scary.

I also get the feeling that if the Echo was Zoidark's form of Tempering, Emet-Slech would have told us, or made some kind of reference to it. Instead he just says he and the other members of the Convocation of Fourteen were tempered by Zodiark. Actually... now that I think about it, I don't remember Emet-Selch ever referencing the Echo.

Saying that the Echo is Hydaelyn's/Zodiark's form of Tempering also wouldn't match up with how Lahabrea and Elidibus think of it. I can't think of anyone who is Tempered thinking of their Tempering as being something odd. They all seem to think it's very natural and inevitable that it would happen how it does.