Perhaps they didn't have the ability to exploit the Shedai tech themselves but do now with the resources of their allies. Maybe they had the tech but didn't feel secure enough to let everyone know that but do now that they have allies.
Or maybe they just didn't want to reveal that they have Shedai based WMD. In "Paths of Disharmony," the Tholian ambassador said they are a patient people. They seemed to be content to let everyone remain in ignorance of what happened in the Taurus Reach until the events in that book.