https://www.faa.gov/air_traffic/technology/rwsl
From what I've found it seems like the indication of the RWSL should match the clearance from ATC. This to me suggests a ground vehicle is permitted to request:
> • DO NOT proceed when the Runway Entrance Lights have extinguished without an Air Traffic Control clearance. Runway Status Lights verifies an Air Traffic Control clearance, it DOES NOT substitute for an Air Traffic Control clearance.
> • If an Air Traffic Control clearance is in conflict with the Runway Entrance Lights, do not cross over the red lights. Contact Air Traffic Control and advise that you are stopped due to red lights.
The page never directly states it¹, but the implication is that a ground vehicle can request clearance, and the clearance from the ATC can be granted, & RWSL should match. If they do not match, red RWSL prevails, and green RWSL are not a substitute for clearance.
Truck 1 did request clearance. So then the question for this incident would be "what was the status of the RWSL when Truck 1 entered the runway?"² If they were red, according to the linked page, Truck 1 should not have entered the RW regardless of the clearance, and the mismatch between ATC verbally granting clearance & the RWSL system seems problematic. But, I don't know what the actual status of the RWSL was, so.
… hopefully, the NTSB report in a few months will contain an explanation.
¹I am treating this page as a non-authoritative explainer, not as legal regulations.
²The one video I've seen of the incident is not clear enough for me to make out the RWSL.
I read a theory that there's only one radio in the truck that needs to be switched between frequencies.
Presumably the truck driver got clearance from the tower then immediately switched away to the frequency that the firefighters use to coordinate.
Just to add…
The vehicle in question got clearance from ATC to cross the runway. ATC revoked it shortly afterwards (by radioing "Tower, Truck 1. Stop truck 1. Stop! Stop Truck 1, STOP!" (followed by the incident; the next transmission is go-arounds.)); presumably, ATC realized the impending danger. I am assuming that requesting permission from ATC to enter a runway in an emergency is a permitted action, so RWSL aren't going to prevent this type of incident.
I don't think we know why Truck 1 did not heed the stop warning (e.g., if it came too later, got lost, etc.), but I am thinking that if they understood the indication from the RWSL, they overrode it by getting clearance, because they needed to cross the runway due to the (first) emergency.
So, same. Will be curious to see what NTSB says. I suspect something about resource management: there seems to be too much happening, too quickly, for that one ATC controller. While perhaps the controller makes mistakes, the mistakes appear to my untrained ear as "reasonable", and I'd like the system to be such that reasonable mistakes don't cost lives.