How can an active rectifier block current?
You are correct, the active rectifier has no way of blocking current. A safety device needs to be provided separately to protect against short circuit. Passive rectifiers have the same problem. 

The MOSFETs are there not to boost voltage but to get better efficiency. MOSFETs can have a very low on resistance sometimes as low as tens of miliohms. An average diode has a forward operating voltage 0.6v to 0.7v and if passing 10A the power dissipation is 6W. If the MOSFET has a RDSon of 20mOhm then the power dissapated is 2W.