it takes some time to update the ATIS and so on to go to Low visibility / CATIII operations.
additionally the ATC may have believed the fog/clouds to be temporary which i believe it was. (also maybe not quite as temporary as believed)
also with approach briefing and sequencing once the system is running and suddenly the visibility gets worse you can't use the landing time any better than letting people try the approach and inform them about the visibility best you can.
maybe they get lucky with the fog/clouds and are able to land.
else they just go around.
should be perfectly safe to do so. (downside is maybe they need to stop departures so departure control can handle the go-arounds)
also in this case the actual RVR seems to have remained above 6000 so once you break out the conditions are quite ok, that is if you break out.
(reason i say this they would have annouced them otherwise)
also you should listen to some seattle CATIII approaches where some airlines don't even take off anymore (below 600ft RVR) but others continue to land and take off down to 300ft RVR i believe. (should have made a recording i guess, was somewhere around the bad fog 8 Dec last year)