Yeah, never heard it banging around. You are right about the crack growth. The insurer's surveyor's determination just does not make sense.
BrainW commented that maybe backing up with the centerboard stuck could cause the damage, which is very plausible. The possible causes are infinite, but not likely a result of wear and tear by the centerboard.
I have not pursued Hunter as much as I probably should. To be honest, I was disappointed in their initial response. When this first was discovered, I contacted them. I left a message with the repair/warranty guy asking if this was common or had been seen. The person left a return message saying that we needed to grind the crack and fill fiberglass. (Not much help.) A Hunter dealer told me that Hunter pretty much washes their hands of hull damage after the 5 year warranty expires, so I haven't pursued them further. A couple of more phone calls to them would not hurt.