You need some sort of chain of trust to do anything big in the world. Im sure their hierarchies are pretty flat to keep the risk of infiltration low, but still, if you have a trusted source that supplies your equipment, then that can and will get compromised.
They might have been checked at some stage, but if you have enough intel, you can figure out at which stage to insert the vulnerability. But i have seen lots of fun ideas on how this could be done. The fact that its apparently alkaline batteries, makes it interesting tho. Unless it was fake batteries with something inside them.
You could also try to make it so that the people who check the devices, only get to see good ones.