Actually its really most likely yet another impasse within the story team and the main dev team where they understood where the 6.X series was going to go but the trial series team really wanted their story to focus on the 4 Elemental Fiends because cool FFIV reference. And since the story was taking place within the 13th alongside whatever their intended version of the MSQ, what prolly happened was a compromise between the team in integrating the trial series and its msq at the same time.
This wasnt the first time, the FFXIV devs had conflicts with where they intended to do stories such as the lost Garlemald expansion that was supposed to be Shadowbringers. Of course this is all just speculation, but its the theory that makes the most sense to me considering how they develop both story and content. Their process tends to be an organized mess.
