The Duke wasn't mistaken when he raised the matter regarding the Manna trail.
As had already been said, Manna trail was a powerful spell that only a handful could conjure. The Obelisk would surely be interested in acquiring the Sorcerer who had conjured it for themself.
Be that as it may, Duke Brahm Gertrude wasn't planning on divulging the caster of Manna trail. He was aware of how the Obelisk operated, so he was certain that they would try and track Ilya down.
Most importantly of all, he made a promise to Ilya that he wouldn't reveal her identity.
{ "Manna trail? And dare I ask who the conjurer was?" }
It was evident in the tone of the Sorcerer on the other line that his interest was piqued. That was as good as acquiring the assistance of the Obelisk, considering that only Meisters were allowed to take calls from nobles.
In a way, that was a preemptive measure to minimize the unwarranted tasks taken by the Sorcerers of the Obelisk.