Visibility can change rapidly particularly with fog banks as we all know. If the runway was unprotected and they continued manually it is perfectly possible, that the approach may at the critical point have been legal to CAT 1 limits rather than as reported above. Continuing when visual reference is then lost has been a factor in a number of recent incidents.
If we line up the holes in the swiss cheese in one possible scenario: The crew are tired, weather is marginal CAT1, the auto-pilot does not perform as advertised, they take over manually and lose visual reference below decision height. If in doubt go-around is almost always the learning point from these incidents.