Wild theory: just like Solitar taught Macht human magic, she used him to learn demon magic. Solitar believed she could defeat Macht, meaning she had a countermeasure for Di Agolze. Ergo, she knows (a weaker version) of Di Agolze, and will use it when she inevitably returns.

Solitar is the key to everything.

If the mangaka is smart he'll make Solitar into the main villain of the story. She's so good.