The results of the bombs definitely amount to mass injuries. Does ''mass destruction'' have to be on a more fatal level? (Since mass murder at least in terms of shootings is defined as four or more victims.)
No one is disputing that the bombs caused a great deal of damage, injury, and death. It's just that when you use the term WMD it evokes a certain image based on its traditional usage, and that is of nuclear, biological, and chemical weapons that have the potential for wide area destruction and casualties measures in the thousands or more, as opposed to low yield explosives with a relatively minor blast radius.
However, while it all sounds a bit sensationalistic, I don't believe there's any intent to fool anyone, and it wouldn't work for the vast majority of the public who knows what kind of bomb was used anyway thanks to all the media coverage. The only people who might be fooled are those relative few who confuse Chechnya with the Czech Republic.
Also, there's a longstanding legal definition of WMD that is being used in the charge here (and has been used before, such as after the Oklahoma City Bombing with Tim McVeigh) that is separate from the popular usage of the term WMD.