I'm unsure why everyone keeps pointing to the three witnesses on the ground who state the vis was only a matter of feet. I can't begin to count the number of times I have hiked in vis measured in metres horizontally yet I can clearly see the sun or the times I have flown over 20-30' layers of fog. Those people on the ground are relevant only as far as the height AGL that the fog/cloud reached. Above that the Chinook could have been in clear air... Sort of like the Yachtsman described... Thus precluding the requirement for IFR and a climb to SA and taking many of the airmanship arguments out of the mix too.
But we'll never know how relevant they are, will we?