Sylas activated Semi-Reversal.
He could use the Scorned Wraps to undo things related to the system. But something like a permanent change to his own body or that of a Contract didn't fall into that category.
He could do it with the Frostbane Feather specifically because it was an item that permanently linked a target with the system by making it a Mount or Combat Mount.
However, Semi-Reversal was an ability of his Vipermancy Profession—one that he rarely used because he didn't often make mistakes, but an ability nonetheless.
It allowed him to backtrack on evolutions he had triggered in his Serpentes, allowing them to reverse to a previous state.
Of course, by now, he had a full Reversal ability as well on top of Semi-Reversal. But he hadn't yet allowed the evolution to go far enough to need it.
The beauty of full Reversal, though—and the irony for that matter—was that Sylas had the ability to revert all of his Contracts back to Level 0 if he so chose.
