The issue is if they want to conform at higher than HD, Resolve cannot recreate all the effects done in the sequence. If HD was adequate for the final master, then they could certainly conform in Media Composer (no symphony needed for this) and send off to Resolve for color correction.
If that is the workflow, then you want to make sure you have all your camera originals conformed without any LUTs or color influencing going on the shots being sent. Also, DNxHD is a Rec.709 codec that can contain full swing of black and white levels, but may not be as robust as going back to RAW sources, or 12 bit LogC 444, etc.
Michael