In my view, what is illegal is to continue the approach below the da(h) if no visual reference is acquired by then. METARS (and TAFs) are for flight planning purposes only. And it is ok to plan a flight to an airport where u know there’s little chance to meet the minimas on arrival providing you choose not 1 but 2 alternates at destination. Now if on actual arrival you don’t get the matching RVR, you can still fly the app to the da(h) with a high chance of GA. If the reported RVR is just so so, there is still the option of GA and entering a hold to wait for an improvement (within the limits of the fuel planning). In this case, they may have had that in mind but eventually became visual by the da(h) making the landing possible