@brandonk Those topics will only work for outputs that are not configured as motors. There is no way to set pwm to thrusters directly via ROS. You may only set the desired roll/pitch/yaw and forward/up/down.
You can check and see when we will fix this here: https://github.com/bluerobotics/ardusub/issues/166
There is a hack you could do and use the MOTOR_TEST command via mavlink. The command is meant to be used for motor testing, but the logic is almost the same. This will require a pretty high message frequency though, so try higher baudrates.