Dort both affirmed the truth and "rejected the errors" of the Arminians.
Heresy is false doctrine, contrary to the dogma of the church. Dort sought to express the faith of the Reformed Church with respect to the five points of the Remonstrants: they condemned their doctrine as false.
I am not aware that they used either "heresy" or "heretics," but that's unimportant. That specific word was often not employed when condemning false doctrine but only in retrospect (as when a later church council would refer to earlier condemned parties as heretics).
Peace,
Alan