Actually I dont believe that this is correct and I am surprised the Drake's would write this. Probably ALL ties have one continuous thread joining the tie together. A slip stitch when you extend that thread past where the tie needs to be closed and then come around looping the thread back . You sometimes see it poking out from the bottom of where the tie is closed. Like a noose. Its purpose is to allow for greater and more free movement of the tie so that it doesnt rip when the tie is pulled up and down etc. See more in David Hober's excellent (as usual) instruction on tie making here.
The proper way to do it is to actually sew it into the interior of the tie (as shown in the picture posted) where the two folds overlap in addition to the "four Corners" of the label or keeper.
I was always under the impression that the purpose of the bar tack was an added (and possibly not necessary) touch to show that the tie will stay sewn closed and not to keep the interlining in place. I think that the presence of of one in an unlined tie as well as the absence in many ties (and the lining still stays in place) bears that thought out.