For me, theres no clear answer.
The Cant used doesn't match the Cants of Calling or Compulsion exclusively, but I'm leaning towards some kind of minor Compulsion to puppet him to the meeting. Fuck it, I'm calling it a Cant of Locomotion and saying that it's never used again in the series to put my OCD at rest.
What I can't understand is using sorcery in Sumna, even hidden by a hood or facing a wall or whatever, seems totally nonsensical.
Given that there's ideas in TDTCB that never fully developed is this one of them? One off spells we never see again before the magic system was set in stone? Again, Inrau's superhuman speed, exploding fiery blood spells etc come to mind. There's also the Nonman "levitating" back to his feet in the initial fight with Kellhus, something that's never seen again.