How does what Trump said make him a “Truther.” You have a warped sense of what a “Truther” is.
A “Truther” thinks that Bush deliberately planned 9/11. Trump merely retorted Jeb’s claim that W kept us safe, by pointing out that 9/11 happened on his watch. He did not imply that Bush deliberately perpetrated it.
Trump has also blamed Clinton for 9/11 as well.
Trump=truther
An interesting take on Bush mistakes over 911 and subsequently is Richard Clarke’s book Against All Enemies. Apparently his warnings to the new Bush administration were pushed aside immediately prior to 911.
https://en.wikipedia.org/wiki/Richard_A._Clarke