Understanding e-bike power/torque
Very quick answer - the torque numbers that are quoted are maximum torque values which do not correspond to maximum power.

Power = torque × rotational speed, so, at slow speeds the motor unit can provide more torque (and hence more acceleration) while staying within the power limit.

A bit more:

There a decent page here on the characteristics of DC motors. The essential takeaway is that DC motors can provide less torque the faster they spin.

Re:


  [I] thought there was a linear relationship between power and speed.


The power at constant velocity is that needed to balance the forces resisting forward motion - rolling resistance and aerodynamic drag for a bicycle or other vehicle. Those resistance forces generally increase with the cube of velocity.

If you want to play with the numbers there is an interactive calculator here.