Is a tilted drywall joint at an angled roof intersection to be expected?
I'm assuming the drywall at the top was not installed by the contractor, and he simply added the new drywall against it.  In this case his point is definitely valid, old drywall is going to be tough to butt up against, especially a slanted ceiling.  I'd just paint this and move on with life.  There is nothing structurally wrong done here, and I highly doubt anyone is going to notice, much less make a purchasing decision based on this!