It's true that half the class is structured around treadmill intervals, and there are blocks and intervals designed for runners. But you don't have to be a runner to get in a good cardio workout at Orangetheory. In fact, the treadmill portion also has a power-walker option where you walk at a brisk pace but increase the incline during intervals.
If you're not into the treadmill at all, there are other cardio options that are available: an exercise bike and a strider. Just be mindful that there are only a couple options for those cardio machines, so if you want to claim them, it's best to arrive early and tell the front desk associate that you need a bike or strider card.
Image Source: Orangetheory Fitness