My Brother-in-Law Elimelech was kind enough to copy the relevant page in Rav Scheinberg's Mishmeret Chaim that I mentioned last time. Here, we see his starting point is the Gemara in Succa: Why is one who is walking to learn Torah patur, while one who is already learning isn’t?
Rav Scheinberg bases his chiddush about the small granularity of the mitzvah of Talmud Torah on the Vilna Gaon(שנות אליהו, פאה). The Gaon wants to explain why one stops learning Torah to do a מצוה שאי אפשר על ידי אחרים, despite the fact that העוסק במצוה פטור מן המצוה doesn’t apply to Torah learning. He explains that it’s a question of which mitzvah is greater. Torah learning, of which every letter is a separate mitzvah, is the greater mitzva and therefore takes precedence.
Rav Scheinberg takes this idea of one mitzvah per letter and applies it to the other side of the equation. Why doesn’t העוסק במצוה פטור מן המצוה apply to Torah learning when it does apply to walking to learn Torah? Because the small granularity of the mitzva means that you're not really in the middle of a mitzva and therefore not patur(compare with R. Lichtenstein's answer to this question, here).
This extension of the Vilna Gaon’s principal is a chiddush in its own right, so it’s not surprising that Rav Lichtenstein quotes the Vilna Gaon(to strengthen the question of why העוסק במצוה פטור מן המצוה shouldn’t apply to Torah Learning when it applies to all the other mitzvoth) without drawing the same conclusion.
Same Principal or Different Principal
It's informative to compare the understanding of the Gaon to that of Rav Lichtenstein. The Vilna Gaon explains that one stops learning Torah for a מצוה שאי אפשר על ידי אחרים, not due to העוסק במצוה פטור מן המצוה but because of another principal where we compare the relative weights of the two mitzvas in question and choose the more important one. Rav Aharon, on the other hand, explained that this case is part of the general rule of העוסק במצוה פטור מן המצוה.