It is, unfortunately, total tosh - unless you want to believe that 'traditional' means 'something someone made up and employed our grandparents to knit so they could sell them'.

The whole concept of the Arran sweaters (I think it was originally Arran) was 'developed' by a man wanting something to sell to the tourists.

The bainin yarn might well have been in use before, for making sweaters much like the fisher ganseys which seems to have been the standard garment for working men - its place, chronologically, being somewhere between the traditional smock and the short jacket.

The white versions were worn for church, though incoming ministers not knowing the local ways were responsible for getting rid of it in some places as they saw it as not respectful and declared the the men should wear shirt and jacket. Its place as a garment for the groom, and for first communions, lingered on, with grandma's embarrassing their grandsons with gifts of lovingly crafted white jerseys for decades.