I interpret this “glance into the pure in-itself” as the subject being apprehended as the object. That’s what I take Kant’s process as, treating the subject as our object. But then the subject is taken to be a part of the “in-itself”, that’s the presupposed glance into the in-itself. You say it this way: “He is referring to the fact that the subjective is a moment within the objective”. Anyway, the result is that the subject, as object (or moment within the objective), cannot escape the indeterminateness which has already been assigned to the object.
In the categorical achievement it expended and impoverished itself; in order to be able to determine, to articulate what opposes it, so that it would become the Kantian object, it must dilute itself to the mere generality for the sake of the objective validity of that determination, amputate it from itself no less than from the object of cognition, so that this would be reduced to its concept according to program. The objectivating subject shrinks down into a point of abstract reason, finally into the logical non-contradictoriness, which for its part has no meaning independent of the determinate object. The absolute first necessarily remains as indeterminate as its opposite; no investigation of what is concretely precedent reveals the unity of what is abstractly antithetical.
I agree that the issue of the object itself, in its purity, is not addressed here. What is criticized is the idea of the subject as object, or the objectifying of the subject. I think Adorno says this is true in the sense that it destroys the illusion that we could have absolute knowledge of the object, by reducing knowledge of the object to knowledge of the subject, thereby imposing a significant restriction on the extent of that knowledge. But it is untrue because it casts the subject as our object, and this imposes an unreal restriction on the object.
The demonstration of this untruth is the truth of post-Kantian idealism; this latter however is in turn untrue in its equation of subjectively mediated truth to the subject, as if its pure concept were being itself.