They don’t mention that non-GM crops, such as grains carry a “lethal toxin” called aflatoxin. Usually it is in low enough levels that there are no serious problems but occasionally when conditions are right things get nasty, or in places where grain storage is not optimal such as in Africa. Old pancake mix can really make you sick, for example, it’s best to keep it in the fridge and use it quickly. Aflatoxin’s been a problem for centuries, and is a carcinogen that Saddam Hussein was working on, though it does not kill quickly enough to be an effective weapon in war. It is a pretty good weapon for revenge or blackmail though. Occasional outbreaks of aflatoxin in pet food, especially ground baby bird food, occur, and isn’t discovered until all your baby parrots sicken and die for no apparent reason.
I doubt aflatoxin has anything at all to do with GM.
It's caused by growth of a fungus, many species of the Aspergillus genus, and is caused by improper storage. Sometimes it grows on crops in the field due to weather conditions, but I don't think GM crops are likely to be any more resistant than the traditional varieties.
BTW, most foods we eat contain some level of mycotoxins.
Since the acute health effects of aflatoxins on adult humans are not particularly drastic, it would not be particularly logical to try to weaponize them.
Some other mycotoxins (fungal toxins) have indeed been researched and even used as weapons.