I don't think we are trying to make the same points here!!

I don't think the History Wardrobe ladies were making the claim that this was a 'universal' or even the most common use of the flannel waistcoat--only that it was a possible use for it, and something Marianne obviously never even considered.

I think it's clear enough that the flannel waistcoats were used by the military (I also found several references to men in the navy wearing them) and the Smollett quote shows that this would be used for hunting garb. While Marianne may never have had any contact with either military officers or men who hunt prior to the events of this story, it's understandable that she would make the other assumption about illness. She may very well have been entirely ignorant of it. For all that Marianne thinks her own views of the world are superior to those of others, she goes with the 'obvious' supposition.

However, my point is not that this was universally done or well known and accepted by everyone--only that there were groups of people who did wear flannel waistcoats for reasons other than illness, and that Colonel Brandon fits into these groups (military, and likely a sportsman). Men in the military and men who hunted were not exactly small enough groups that this would have been a secret !