No it doesn't mean that at all. The UN had tagged and sealed the Cemtex explosives in January. When they returned they were looking for WMD not Cemtex which they had already found and sealed. The white powder story is from FOX 18 months ago and nothing ever came of it.I'd think if the white powder was WMD we might have heard about it before this.
I agree that the context indicates that "nothing" in this story refers to WMD, not what they had sealed and tagged. However, the "white powder" was determined to be run of the mill explosives and was not the 380 tons of HMX and RDX that is the centerpiece of this attack on President Bush. When our troops arrived, the 380 tons was missing.