Off-road or not, they were driving very slow on the pavement, as if they were positioning.
Probably because they had been advised that the plane had already radioed in to say that emergency services were not required on landing. At that time, little did anybody know that just after touching down a fire would start. Maybe with an update that the AFS were already "in action" on the ground, the decision was made that it would be safer to keep all PAX onboard so that the fire could be tackled with maximum effect and without having to worry about stray (and possibly disoriented) people around the plane.
Full details of the whole drama are needed before we can criticise or commend those on the flight deck for not ordering an evacuation