I mean, isn't it obvious that they didn't?

It’s obvious that either they didn’t, or they did but they didn’t see the plane. We don’t know which.

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.

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.

I have over 1,000 hours as PIC in the NY area alone, in all seasons and all weather, and I was in the region last night, awake when the accident happened. LGA was reporting 4SM -RA BR FEW045 BKN090 OVC110 at the time of the accident. The weather wasn't anywhere close to what I'd call "bad" here. Sprinkles and some high clouds.

I can almost guarantee you the airplane was visible from taxiway D.

You can also see from the videos that they could have easily seen the plane.

I want to agree, and it almost certainly seems like the case but visibility can be vary widely by position. So just because the camera was about as far away from the collision as the plane would have been when the truck was entering the runway, it doesn't mean the plane was visible to the firetruck.

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.