In fact, Trump has not claimed that Bush had specific knowledge of the 9/11 attacks. He said, “George Bush had the chance, also, and he didn’t listen to the advice of his CIA.” That is correct. Bush was given numerous, detailed warnings that Al Qaeda planned an attack. But the Bush administration had, from the beginning, dismissed fears about terrorism as a Clinton preoccupation. Its neoconservative ideology drove the administration to fixate on state-supported dangers — which is why it turned its attention so quickly to Iraq. The Bush administration ignored pleas by the outgoing Clinton administration to focus on Al Qaeda in 2000, and ignored warnings by the CIA to prepare for an upcoming domestic attack. The Bush administration did not want the 9/11 attacks to occur; it was simply too ideological and incompetent to take responsible steps to prevent them.
It is possible for someone you despise to say something you agree with.