I am not too sure that I understand what you mean by the phrase "in-depth psychological study" of Heathcliff. While I think the novel does provide us with excellent insights regarding Heathcliff's character, if you think that this phrase is talking about a kind of investigative presentation of Heathcliff where we understand his character completely, then I would have to disagree with the statement, as, by the end of the novel, we as readers are still left with so many questions and uncertainties about him.
Consider first of all his obscure origins. His entrance into the novel comes as something of a fairy talle, as he has clearly mesmerised Mr. Earnshaw and broken the toys that he brought for his two children, symbolising the cuckoo-like way that he supplants them in their father's affections. We are given an understanding of how the abuse he receives at the hand of Hindley could be responsible for his later actions, in particular his desire to get back at Hindley, and how his loyalty to Catherine is tested by her growing friendship with Edgar.
However, a crucial gap in the information we are given about Heathcliff is where he went after hearing that Catherine was going to marry Edgar and what he did. We do know he comes back transformed physically, and Nelly suspects he might have been a soldier. However, he comes back determined to gain his revenge on Hindley and Edgar for the loss of Cathy, and pursues his aim with such ferocity that he destroys many in the process. So often, his evil acts are just hinted at rather than explicity explored, such as his son's references to his father's abuse of him to the younger Cathy.
Therefore, given the number of gaps surrounding Heathcliff's life and narrative, it is hard to say that we are given an "in-depth" psychological study of this important character, and by the end of the novel, I think it is safe to conclude that we are left with more questions than answers about Heathcliff.