Yeah, the Ascians probably weren't behind Susano's summoning. It feels way too random for that. The way Koji has talked about it, it was something nobody planned for and happened by chance. The event that set off the Susano summoning was getting all three artifacts in proximity to one another. Which leads me to believe Susano could have been summoned earlier if the artifacts hadn't been split up... And he's also one of the few primals we've seen who hasn't tempered anyone by accident or on purpose.