In the VehicleTypeImpl class, you can find the following:
* default cost values for default vehicle type
private double fixedCost = 0.0;
private double perDistance = 1.0;
private double perTime = 0.0;
private double perWaitingTime = 0.0;
private double perServiceTime = 0.0;
Thus, when you do not set time cost for the vehicle type, and in the cost matrix you add time only and no distance, your route will always get 0 cost and will not be as expected (or as you said, not as accurate).