The thing-in-itself was probably the logical structure of old Metaphysics. It was a nod to it. Remember, old Metaphysics was not just being as things but being as intelligibility. Purpose and so on. So there was a logic from prime mover which connected things through cause. To know the causes, not just the things. The point or purpose of being. Final ends. Now Kant was saying, in the Critique of Pure Reason, that may well be, but we can't observe it. We are on more sure ground not to speculate about it, and just to take up what appears. A very conervative approach. With Kant the necessary connections are not in the world but in us, Copernican Revolution so-called.
We see Hegel going back to a grand Logic (though the Logic was unconnected to the world it was a necessary ground), and even a telos of a sort. There was a necessary intelligibility again, if taken as a whole. Speculative philosophy.
As for Freud, he was a scientist, an empiricist, primarily in that he observed the patient's mind unfolding during therapy through his method (free association, etc) And he met with other psychoanalysts to discuss their observations in regular meetings.
And Freud was very well read, and I am sure Bretano left a large impression on him, as he did with Husserl. There are surely papers on this. So Freud was a combination of his time and place, his reading and study (Eduard von Hartmann?), and most importantly, his scientific observations.
P.S. The YouTube video you watched may make some very good points. Note, Eduard von Hartmann wrote this book: "Das Ding an sich und seine Beschaffenheit ("The thing in itself and its nature", 1871)" The Wikipedia is not very complete on von Hartmann, German Wikipedia could be better. It does say that Jung claimed to have read von Hartmann "assiduously". It says von Hartmann influenced Freud but gives no specifics. https://en.m.wikipedia.org/wiki/Karl_Robert_Eduard_von_Hartmann
This may explain how Freud was introduced: philosophy of the unconscious. "The work was widely read. Philosophy of the Unconscious received a critical discussion in the philosopher Franz Brentano's Psychology from an Empirical Standpoint (1874); Brentano commented that Hartmann's definition of consciousness perhaps referred to "something purely imaginary" and certainly did not agree with his definition of consciousness.". https://en.m.wikipedia.org/wiki/Philosophy_of_the_Unconscious
So this empirical emphasis from Bretano and the work of von Hartmann could have provided a lot of material for a mind like Freud's.