I think it means that if the trading of bullets is methodical and non-chaotic it shouldn't be Sieze by Force. Imagine two forces both entrenched and neither attempt to storm the other. Both forces are equal sized gangs with 3-harm and 2 armor thanks to their armor and sand bags.
MC: So do you roll out any battle plan?
PC: Nah, I'll just let the days go by.
MC: Are you present, like, taking part in taking potshots and keeping watch?
PC: Yeah of course.
MC: Alright. A few days go by. The most exciting thing that happens is your Baka managed to shoot some assholes hand off when he was giving you the finger. Day 2 a stray bullet passes through your helmet when you're keeping watch, that's 1-harm and roll to see if it bit off more than part of your ear. A few other people get similar boboos, Jake-Jake gets her nose blown off so now she looks even more like a grinning skull.