HARM is a category of weapon which seeks things like radar or jammers. They weren’t suggesting that the jammers are literally harmless.

In unrelated news: the jammers are, in fact, harmless unless you’re making a habit of riding on top of the tank. The radio energy isn’t going to penetrate a significant thickness of conductive material, such as armour plating. Or unless you’re the person being jammed, in which case they’re a different category of harmful