Yep. Exactly as I anticipated that it happened. The Controller clearly forgot about Air Canada and that he had cleared him to land. There is no practical reason that Truck 01 and the accompanying vehicles would have ever had time to get fully across the runway and out of the RPA.
Major screw up by the Ground Controller that tried to correct his error at the last minute and simply could not get Truck 01 to stop... It's also obvious that the second vehicle in the convoy either saw Air Canada, listened to Ground Controller frantically telling Truck 01 to stop, or both.
Tragic for everyone involved. Those firefighters were incredibly lucky that the plane hit the back of their truck and that the cab was was spared. Borderline miraculous.
The firefighters were not at fault here, although I will say that their defensive driving and situational awareness was terrible here. Had they looked left and right down the runway, per the rules of driving on the airfield, they would have seen the lights of Air Canada barreling at them and they could have held position. It's evident that they trusted the Controller's clearance across Rwy 04 and didn't look prior to crossing. For the same reason that you are supposed to look left and right at an intersection even if you have the "green light", we do the same on an airfield despite the verbal "green light". Controller's fault, firefighters didn't do their part and help the situation.
Good for the PIC flying the plane to not deviate off the runway centerline in any frantic way. Very wet conditions at relatively high speed could have led to the aircraft cartwheeling. Had they deviated to the left they would have also collided with all of the vehicles lined up on Twy D and there would be a lot more dead people. Instead, they did what they're trained to do in that instance, which is steer straight and use the cockpit to absorb the impact, protecting the wings and the belly of the aircraft from absorbing contact and igniting the fuel in them. Wouldn't be surprised if the pilot flying was former military. That was a painfully disciplined decision to sacrifice themselves for the sake of their passengers and the safety of the other people operating on the airfield.