Jackie
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.
Yes.
a. Mainly because aircraft cannot get out of, or into, other airports.
If an aircraft is having trouble with (say) LGW earlier in the day, it is not going to be at AMS for the evening rotation - or even for the next morning's rotation. And some Lo-Co airlines are renowned for not having any spare crew, when the previous crew go out of hours, and so the next AMS rotation is cancelled - but that has nothing to do with AMS inbounds.
b. Some airlines still insist on buying Boeing short haulers, which cannot land in thick fog (only Cat IIIa). Again, nothing to do with AMS.
The holding patterns are rarely used at AMS, which makes it a bit of a shock when ATC suddenly says 'take up the hold'. The main delays are very strong westerlies at AMS, because it is then down to one landing runway. Luckily that does not happen too often.
Jackie
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.
LGW have a 'follow the greens to the stand' system. Its a doddle.
.