An entry in his log exhibited that Mahathir actually bought into the 9/11 conspiracy theory.
I could come up with two reasons:
1. He's trying to gain popularity on a global scale with his Muslim brethren.
2. He's forgotten how people could twist the truth like the Fake moon landing conspiracy theory or the Cydonia face alien legacy.
Granted, there are some mysteries to be explained. But for those who claim they have solved it, much is needed to be answered.