The Minhag is not to learn erev Tisha b'Av after Hatzot
Even when Tisha b'Av is observed after Shabbat. In that case, one may eat meat and drink wine until sh'qiah, yet not learn Torah! How so?
Here is my approach
Apparently it's due to the nature of Eating vs. Learning Torah
The pleasure of eating meat ends when one eats it. While digesting it, it is no longer such a [significant] pleasure.
OTOH re: Torah, while it is being "digested" and absorbed, the afterglow Simchah remains for hours. Thus, even on Shabbat one must refrain from Hatzat lest one STILL enjoy that Torah Learning after Tisha b'Av has commenced later on that evening
Kein Nireh Lee