I think there is no doubt that Marianne assembles the evidence and comes to the conclusion that suits her ;-)

But there is no doubting the evidence ,which is copious, that the use of flannel garments was associated with the infirm in the 18th century and later,and she is drawing her concusion from that association...

It may be wrong of her to dismiss other causes of his use of flannel wasitcoats but it us perhaps typical of teenager to come to the obviousrather than the subtle/hidden conclusion.

My post was meant to show that the evidence is there to support her statement( but I confess the assertion by the History Wardrobe, and any supporting evidence, has been rather more elusive,though I am still searching for it, even to the extent of trawling through Henry Angelo's copious memiors!)