"It looks to me as if you are to seam from the top edge of the neck ribbing to where the shaping begins. This would give you a tube that ends where the angle starts. The seam would be on the bottom, between the dog's legs, the angled part would be flat to cover more of the back."
Yes, that's what I would do too. You only need the seam from the beginning of the neck to the cast off of 6 (9,12,14)sts. Then it's open from there on till you get to the sts on the needle. They're being held for the trim or ribbing which is knit in the round. It's going to be a great looking dog sweater!
(If you can delete page one of the pattern, it would be best. It'll help us avoid copyright issues.)