I had a little spare time today, some of which I spent carefully re-reading the in-progress draft of Hilde Thunem's essay on the Viking apron dress.
Ms. Thunem summarized Inga Hägg as reporting that the Birka finds show evidence of two different ways of sewing loops for apron dresses. One, the technique I use, involves folding the strip of cloth that is to become the loop in half, folding the edge of each half inward, and then whipstitching the doubled edges together. Loops made with that technique were found in grave 835 at Birka, and also at Køstrup in Denmark; I wrote about the Køstrup find, including its loops, here. The other technique involves bending the edges of the strip toward the back of the strip until they meet in the middle, and stitching them together there. A loop made using that technique was found in Birka grave 465.
What this evidence shows is that, although my way of making loops for apron dresses was used, it was not the only method. Perhaps each seamstress used her own preferred method; the fact that edge-sewn loops were found in both Birka and Køstrup suggests that that may have been the case.