Okay so how Transcendental Object is defined as Concept of an Object = x where x maybe any property and thus it can represent any object of realm of appearances. This object posits rules for the synthesis of the manifold. But there is also this assertion of Transcendental Object and Transcendental Subject ( whose job is the synthesis) make each other possible. In essence these both are in mind , and them both need object external given which could affect the faculty of sensibility. If this external object is thought to be same as the transcendental object X then it'll be the object of sensibility positing rules and thus we can't blame Kant of Dissertation for implying that concepts are applicable to things in themselves a priori.