The idea is to seam up the mitts but leave an opening for the thumb. You could sew up to where you'd like the thumb opening, fasten off the yarn and start seaming again above the opening.
Instead, the pattern suggests doing the blanket stitch along one side of that opening, seaming above the opening then going back over that seam to the other side of the opening and using blanket stitch on that side. That'll make the two sides edges look the same at the thumb.
You could always just seam above and below the opening if you prefer.