Maybe some people confuse the apron of the kilt with a masonic apron?

I see a lot of masonic regalia for sale that goes with a kilt but I can't see why that in itself would cause the confusion.

My late stepfather was a mason and a former lodge worshipful master but he hadn't been active for some time.

An interesting point - as the initiation rite involves the candidate having one trouser leg rolled up there is clearly no provision even in the Scottish Rite for a kilt to be worn.

So I conclude it's the apron, however tenuous the actual connection.