Airbus Girl, not quite sure what you think is wrong with David's explanation. The (proximate) reason why Heathrow is in the state it is in right now is the impact of fog on visual contact with other aircraft. As he says.
(Lots of wider issues of course - insufficient runways / too many aircraft; EVS not deployed and not likely to be anytime soon; ADS-B ditto; HUDs underutilised; no mixed-mode; etc...)