I'm very, very curious about whether the ARFF crew visually cleared the runway and final before crossing the hold short line. It's standard procedure for flight crew to do this, specifically to mitigate the risk of ATC errors.
At night with multiple runways it can be very hard to see a plane on final.
Still, I'm always hesitant to cross an active runway.
Yes ARFF should still look before crossing, but the weather wasn’t great with limited visibility and thus even if they looked it’s possible they didn’t see anything.
Reports are there were fog and rain at La Guardia at the time of the incident. They were on a short final, and it’s entirely possible they were not visible to the fire truck’s crew.