As most people would say, the warranty is very subjective. Try a different dealership. The cooling system IS covered under CPO IF the damage was due to a defect. It's not covered if a rock goes through it, technically speaking.

Would you expect them to cover your hood if a rock hit it and damaged it? Of course not. Same thing with a radiator...just unfortunately, it's a functional part of the vehicle versus some paint damage and a dent.

Again, try another dealer. Maybe one would goodwill this and report it to BMW as a defect.