He's saying all that based on this:
Trump’s Misleading Claim That He Warned About Osama bin Laden
"Mr. Trump’s claim that he delivered a prescient warning about Bin Laden is hyperbolic. His book, “The America We Deserve,” which was published in January 2000, contains one reference to Bin Laden in 304 pages:
“One day we’re all assured that Iraq is under control, the U.N. inspectors have done their work, everything’s fine, not to worry. The next day the bombing begins. One day we’re told that a shadowy figure with no fixed address named Osama bin Laden is public enemy number one, and U.S. jet fighters lay waste to his camp in Afghanistan. He escapes back under some rock, and a few news cycles later it’s on to a new enemy and new crisis.
Mr. Trump continued:
Dealing with many different countries at once may require many different strategies. But there isn’t any excuse for the haphazard nature of our foreign policy. We don’t have to reinvent the wheel for every new conflict.”
This fleeting mention of Bin Laden was not exactly ahead of its time.
The Qaeda leader had for years been
linked to numerous terrorist plots and was regarded as
one of the most wanted terrorists in the world; as Mr. Trump noted in his tweet and his book, Bin Laden was the target of
an unsuccessful missile strike in 1998. CNN
reported in 1999 that American officials feared Bin Laden would plan an attack against the United States; he was
named as an unindicted co-conspirator in the bombing of
the American destroyer Cole in Yemen that killed 17 American troops a year later."