I've been experimenting with this. I think what you are supposed to do is:
Row 1(WS): P3, pulling yarn tight, work to last 3 sts, sl 3 with yarn in front.
Row 2: K3, pulling yarn tight, work to last 3 sts, sl 3 with yarn in back. Rep rows 1 &2 for pat.
This is only different than what you gave in one place. On row 1 which is the wrong side you want to begin with P3 instead of knit 3.
I don't really understand how this fits into the overall pattern but what happens when you do this is to make a sort of I-cord along each side and you need to have it purled at the beginning of the wrong side row to throw the knit stitch side of the cord to the right side. You always slip the stitches so that the strand runs across the wrong side (that part is written correctly), turn to start the new row and wrap the slipped stitch strand around the 3 slipped stitches, pull very tightly before you begin to knit or purl, and pull tightly after each one of the first 3 stitches that you knit or purl at the beginning of each row. That makes the I-cord effect.
If you work it as written one of the I-cords is on the front and the other turns out wrong. I don't know how you were supposed to work the stitches between the two I-cord edges but I did them all in garter stitch and it looks pretty neat.
Try that and see if you think that is what you are supposed to do.