The Clarity's single-speed transmission never comes into play in an acceleration test. It only works when cruising at speeds greater than 45 mph. The Clarity's electric torque comes on at 0 RPM, which is not the case with the Outback. That early jump is probably how the Clarity can achieve a quicker time.
Just to complete the thought on Engine Drive mode, below is a chart from a Honda paper on the Accord PHEV which has the same kind of i-MMD system as the Clarity, and it demonstrates the very limited torque provided by the EV when in Engine Drive mode. All of us who ask for more power in Engine Drive mode knows that it immediately reverts to HV series mode.