On the Weapons Chart there's a Notes column. In that column, for several of the Cone rifle shells, there's an "area" note. ECM doesn't have that note. This is an indication that it affects only the target, much like a solid slug only affects its target.
Think of Star Wars and other sci-fi movies. Every so often, someone will shoot a bot with something which attaches to the bot, and then the bot goes all fritzy, with lots of cool electrical special effects, and keels over. That's an ECM "shell" in effect, only affecting its target.
Famous Game Designers are notorious for their ability to express such complicated ideas by the well-thought out omission of words. It's a knack.