Just out of curiosity, I looked up Rav Shach, and surprise surprise, he says no such thing.
What he says is essentially a Taz in shu”a YD246:4, which puts chochmas hakabalah together with shaar chochmos that one should only learn b’akrai and only after learning kol hatorah kulah.
These shitos hold that kabalah is hakaras haboreh, and not a mitzvah of talmud Torah. Just like many hold mussar is a kiyum mitzvah of v’davka bo, and not talmud Torah, even though its contents are definitely Torah.
Kabalah is Torah but its study might not be considered the mitzvah of talmud Torah in halacha. Many disagree about it and mussar; the shailoh might be תלוי זה בזה.