Introduce a foreign object onto the runway and it will inevitably collide with an aircraft. The fire trucks aren't part of the airport traffic management system, their sudden presence is bound to lead to problems eventually.

It wouldn't surprise me in the slightest if the truck has a single radio (airplanes always have two) and was constantly switching between ATC and fire house frequencies. The probably never heard the "stop, stop, stop stop.."

It would also not surprise me if airports previously had dedicated fire services, which have since been outsourced for cost reasons.

This is an airport-specific vehicle that was on radio with ATC at the time and had clearance to cross the runway. Nothing in your comment is correct.

According to this article, the air traffic controller gave the fire truck permission to cross the runway. So, it seems like they are part of the air traffic management system?