The Maharal in Nesiv HaTorah towards the end of chapter 4 writes that one does not need to make a Birkas haTorah on thoughts of Torah. The Maharal explains that this is because a person is defined by speech, as Onkelos translates ויהי האדם לנפש חיה, Man became a living being, as רוח ממללא, a speaking being. Man is defined by the intersect between the intellect and the body, which is exemplified in the bringing out of the thought into speech, which requires the combination of intellect and action (this is expanded in Gevuros Hashem ch. 28). The intellect, and therefore thought, is not really part of the person (this is expanded on in ch. 9), and therefore involvement in Torah in thought is not really the person himself being involved. It is only through speech that the person is considered to himself be involved in Torah.
The Maharal does not explicitly say that this is not considered Talmud Torah, but the implication of not requiring the Birkas haTorah implies that this is not a fulfillment of the mitzvah of Talmud Torah.