The source says that elevator and ailerons where blocked, so the rudder still could have worked. It also says that after trying out they commanded the aircraft with the engines (but this does not exclude the possibility of partial inputs from the rudder).
According to my knowledge, Avro RJ have mechanical flight controls, coppled to the yokes via cables. All control surfaces are independant, no hydraulic input.
This is a really amazing story, and those pilots really showed their professionalism. Not only big plane pilots can be heroes!!
Dani