The claim wasn't that Hussein had never used weapons of mass destruction. It was that Hussein did not have weapons of mass destruction at the time of the invasion. Remember that the justification for the invasion was that Hussein was stonewalling UN inspectors and that the US had intelligence claiming that he had a supply of weapons of mass destruction. As such weapons would be against his post-Kuwait treaties, that would have been a justification for an invasion.
The Bush administration based its rationale for war principally on the assertion that Iraq possessed weapons of mass destruction (WMDs) and that Saddam's government posed an immediate threat to the United States and its coalition allies.
After the invasion, no substantial evidence was found to verify the initial claims about WMDs. The rationale and misrepresentation of pre-war intelligence faced heavy criticism within the U.S. and internationally.
There are many statements that Trump has made of questionable veracity (e.g. about Ted Cruz's father or Barack Obama's citizenship). That quote is not one of them. That was actually a rather mainstream criticism of the (George W.) Bush administration.