From the cockpit dialog, it sounded like when they were taxiing, they spotted something in front of them that was not in the plan -- that could easily have been the one fire truck from which our hero came. There are plenty of fire trucks following the bus.
Also, when he does come on camera with the hose, the "five-minute wait" is still in effect (it's about 2:30), and someone on the gorund tries to call him off; there's concern that he'll ruin the test. With the fire, and when the guy starts the hose, the folks on the flight deck try to argue that it's "not a serious fire" -- and they can make it. Then the tires start exploding.
From a logical point of view, it would not make sense to give the test pilots veto rights over fire crew intervention. So the crew's call that it's "not serious" wouldn't prevent the fire folks from stopping the test -- and the crew seems to realize this.
At the same time, you get the feeling that the truck wasn't where it was supposed to be, and acted autonomously. But given the facts of the matter: the tires were still inflated, and the test was going to fail spectacularly, the "rogue hose" could have been prescient instead of stupid, and they could have been positioned to deal with the NLG if need be.
Even so, you do have to wonder why there appears to have been nobody whose primary task was assisting the crew with evacuation: for several minutes after ground gives the call to evacuate, they sit up there, asking for the stairs with no response. My personal favorite is:
"Uh, excuse me, could we have the stairs or something please?"