Weapons of Mass Destruction have three major components; nuclear, biological, and chemical. One of the things that these three components share is the ability to create mass destruction from one release.
Just ask the Kurds, Saddam tried to exterminate them with chemical agents. Fast forward to post invasion Iraq.
A combined Associated Press and Fox News report released on May 17, 2004, mentioned a sarin nerve gas discovery. Troops handling the shells that contained this gas ended up getting treated for chemical exposure. The Iraq Survey Group confirmed this sarin gas. Mustard gas was also discovered.
As chemical agents, sarin and mustard gas are WMD. Whether it was pre 1991, or current, is beside the point.
People that insist that President Bush lied about WMD existence argue that Iraq had no WMD. No WMD means zero WMD, regardless of manufacture date.
Even if they try to argue that these WMD predate 1991, they prove their own, "no WMD," argument wrong. By trying to argue that this WMD was old, they destroy their own argument that President Bush "lied" about Iraq WMD existence.
Two Iraqis, Gazi George and Georges Sadda, indicated that Saddam moved WMD out of Iraq into Syria. Gazi George further argued that Saddam was capable of burying his WMD's underground. Gazi George was an Iraqi scientist, and Georges Sadda is a retired Iraqi Air Force general. He had regular contacts with Saddam Hussein.
In December, 2005, Israeli Lieutenant General Moshe Yaalon argued that Saddam moved chemical agents to Syria before the invasion.
Burying WMDs underground, in a place we haven't searched yet, is very realistic. People accidentally discovered buried fighter jets in the Iraqi desert. They also found buried earth moving equipment.
Had the Iraqis did a thorough job burying the MiGs, and had their tail fins not been sticking out of the ground, they still would've been hidden. How could we miss these despite our inspections?
The inspection teams that we sent to Iraq didn't inspect the whole country; but a limited area.
Their conclusions could only responsibly be applied to the areas they searched. Charles Duelfer even refused to rule out the possibility that WMD were moved to Syria.