Hmm, just browsing through the pattern (which is found
here for anyone else that takes a look at this post) it looks to me like it should come out rounded because the decreases are done gradually over 60 rows. It doesn't matter that there aren't decreases at the very edge of the piece; the decreases placed inside the 15 stitch border still makes the rows shorter, but while maintaining the border as it has been up till now.
Sorry I can't be more help.