Liike any other Health, Wellness, and Fitness application on the App store, the Kardia App tracks activity for the amount of time in which you invoke the watch’s workout mode.
Your step count should not be effected, we do not create our own step count, we are simply reading the values of what is already being collected by the Apple Watch, and as you've seen, sometimes it will count steps while you are sitting due to wrist movement. The rest of the data that is being collected is valid, although it effects the expected outputs of the Apple's Activity app as follows.
Move: The Move goal is effectively your step goal, but measured in active calories. While in a "Health and Motion" while running SmartRhythm, additional calories are ascribed to the move goal.
Exercise: The Exercise goal seeks to measure minutes of exercise. When workout mode is enabled, the Activity app is incrementally sensitized to movement.
Stand: The Stand ring considers all workout time as meeting stand time while running.