I apologize, I didn't word that very well. Every muscle is being used, how much is the issue. If you're relying on your arms to move/hold the string, which I have in the past, it's not that the back isn't working, it's just not doing as much as it could. This lead to a lot of problems for me.
In fitness, some folks suggest you can target and strengthen a muscle just by focusing on its engagement more during chain training. While an arm curl as an exercise targets a specific muscle, shooting a bow is like a pull up using many muscles together in a chain. Not as specific. If you focus on the tension in your arms during a pull up, you can turn this "back exercise" into an "arm exercise". The back is still doing work to get you up to the bar, but you can make your arm bear more of that load by focusing on it specifically, which means bigger arms for the fitness enthusiast. That's still not as easy to teach as it is to just say "do bicep curls for bigger arms" though. As archers, we have to learn to focus the load onto the back specifically in the chain.
Hopefully that clears up what I was trying to say. The mental side is probably the hardest part of archery to discuss or explain.