A Book of Tafsir is Not the Same as the Qur'an
Question :
Why is the following verse not written on Tafsir books?
"None can touch but purified"
Answer:
Since a Tafsir book contains words of men, it is like all other religious books: a person who is impure may touch it, even if all or most of the Qur'an is written in it. What is forbidden is for an impure person to touch a copy of the Qur'an, which either consists of the Qur'an by itself or what is for the most part the Qur'an.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 7 Page 130