I'm also genuinely disappointed that I can't turn up with Alchemist to do some Thavnairian quests. C'mon, guys, alchemy was Thavnair's whole thing until basically Shadowbringers!

That said, you do basically see on oblique confirmation that the Totentantz was dynamis-related by talking around Radz-at-Han after 6.0; there's at least one NPC that recaps what the Terminus Beasts were using almost word-for-word the exact same description as the dancers use to describe the Totentantz, suggesting that yeah, it was identical observed behavior.