It looks like that is based on the last recorded speed from flightradar24[1] which was 21kts(24mph). The previous data points were 11kts, and 58 kts(the last point before the track deviates off the runway). I do think it is likely that the collision occurred at a speed faster than 24mph.

edit: Looking into this a bit more it looks like the plane came to a stop around crossing E while the emergency vehicle was crossing at D(based on ATC recordings). Using the following map as reference[2], the 58kts point was around E, while the previous recorded point which was just before D was 114kts.

[1] https://www.flightradar24.com/data/flights/ac8646#3ede6c39

[2] https://www.flightaware.com/resources/airport/LGA/APD/AIRPOR...