I'm not familiar with the Q400 system architecture, but I'm reasonably sure that each engine drives one hydraulic pump (typically the pump on the left engine would drive the right hydraulic system, and vice versa; brakes, flight controls, etc generally have a supply from each system, so loss of one pump doesn't mean those systems are totally inop)

it's ok to continue if one pump fails in flight, but you probably need both operating for dispatch; therefore, they would have had to cancel the xMFR trip since they likely didn't have a spare pump there