The point is that a much larger airport, that is not capacity restricted like LHR, would not have to reduce traffic flows during Low Vis opps. Had this recent fog blanketed a much larger Thames airport, there would have been no flight cancellations.
That is utter balderdash. Look at, for example, Schiphol: 6 runways operating notably under maximum capacity. Fog there today and recently, lots of significant delays and cancellations.
Toronto Pearson: 5 runways, has to cease use on at least on of them (24L/06R) when they are operating in Low Vis. And consequently flow rates decrease.
Detroit: 6 runways, 4 of which are parallel and able to normally operate simultaneously. It is not yet at saturation like Heathrow, but flow rates drastically drop when in Low Vis.
This is because it's all being well landing an aircraft on the runway, but they have to then taxi from the runway to stand. This isn't easy when the fog is so bad the controller can't see the plane, and the pilots can't see much further than the end of their nose.