Moorsel where did you get that information about the GA pilot not having an IFR licence ? According my info both Citation pilots had one, the PIC had a IFR cat 1 with 5000 hours and 2400h on type, the co was ATPL FI , had 12.000h TT and 2000 h on type and an IFR CAT 1 as well.
What you probably refer to is the fact that they were not CAT3 qualified and that the Wx condition soutside were indeed CaT 3 but not reported as such by ATC.
But even then, according regulations nothing prevent you to taxi under CAT3 if you have a CAT 1 licence, because you can always wait for improvement at threshold and take off under improved visibility..as it comes.
So what they did was strictly speaking not illegal.
And again the airport at the time was not declared Cat 3 ops ( read the transcript of the ATIS at the time )
Like in any accident, the answers is a mixure of things and not a single person is responsible for everything.