That particular plane mustn't had been equipped with Cat 3 navigation system.
I've flown HKG-DEL on CX - captain announced our plane was equipped with CAT-3 and landing in heavy fog in DEL was not a problem. Our arrival into DEL was on time with no issues.
Most other planes were diverted and heavy fog was on the news that day.
Not all planes are equipped with CAT 3- computer auto landing.