> I think that's quite a lot faster than most people are watching out for

That isn't even beyond the top speed of a car, which non-trained humans are very well capable of tracking by sight - to talk of airport workers that are specifically trained to look for air traffic. It really is not that hard to tell that an aircraft is on short final if you are actually looking at it.

With four miles of visibility in light rain at night, the aircraft should have been perfectly visible (in a vacuum); what remains to be determined is why the ARFF crew did not see it. The answer to that could range from "they didn't look at all" to "the orientation of the runway relative to the surrounding neighbourhoods meant that the CRJ's lights got lost in the city lights".