Edited out convocation mention in fall of Eth, because the convocation is always suggested as an involuntary explosion using the tower of Redoubt, while the burying of all technology is separate and voluntary in fear of dragon cultists misusing it. The book, otherwise, quoted, also never mentions the convocation as taking place at the same time. The best guess is either the Ethian Empire suffered two destructions of their culture, the first being when they buried all of their technology, forcing them to regain control over the world through magic, and the second being the convocation, OR the Ethian Empire never willingly destroyed their technology and that was just a pleasant lie after the convocation. The latter seems unlikely because it would be pretty hard to be like, "Oh yeah, no, that explosion that killed most of us? Totally on purpose."
Further, the convocation is never mentioned as using sourcestone technology. Only standard fare magic fantasy tower focusing magical energy.
EDIT: ALSO BOOK CALLED SECOND FALL OF THE ETH