Well, but this incident happened at night – 9.26pm, according to post #1.
Often at a busy airport the following a/c lands very soon after the preceding a/c enters the high speed exit. If the preceding a/c unexpectedly leaves contamination behind it at night, then unless its crew knows something's fallen off and calls the tower instantly, neither ATC nor the following crew can be aware of the contamination anyway. OTOH, if the preceding crew know beforehand that their a/c is likely to cause contamination, they'll have said so long before landing, and there'll be an emergency in effect. Although it seems this AC crew wasn't hearing the tower frequency, so if either of the above had happened, they wouldn't have known ...
If there's a vehicle about to make a runway incursion at night then (a) it'd have visible lights, (b) before entering a runway, IMHO any sensible vehicle driver (or pilot) should routinely look for traffic on the runway and up the approach.