According to Rolls Royce's book of the jet engine, the pressure and flow energy multiplied by the area at the outlet is greater than the pressure and flow energy multiplied by the area at the inlet, so there is a net forward force. At high throttle the flow is likely to be at the local speed of sound or above, leading to an increase rather than decrease in speed in a divergent duct, the increase in momentum being reacted as a pressure on the wall of the divergent duct which being angled has a forward pointing component.