The way I learned box pleating from Matt Newsome, the steeking is done without cutting out the pleats - no need to because there isn't the bulk of a trad knife pleated kilt. So, the steeking doesn't do the same thing in a box pleated kilt because there are no cut-out pleats to keep from sagging. The main thing that the steeking in a box pleated kilt does it to provide strain relief across the stitching at the bottom of the fell. in the pics below, the pleat stitching is behind the long horizontal threads, so any horizontal stress at that point is partly supported by the floating thread and not entirely by the pleat stitching.