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.