You missed a couple things. In the scp mythos, Adytum was Ion's capital city thouands of years ago. Also part of his title: "the sorcerer king of Adytum."
Also the ritual wasn't intended to summon Ion. Ion, or at least the guy claiming to be him, was already there. He is the POI they were trying to assassinate.
Also the Demiurge is another name for Yaldaboth, a deity from Gnosticism and in scp the name for the Sarkic god.