I think the only case they might have is that security measures taken (regardless of declared value) were substandard. It does seem strange to me that it's considered ok to leave the truck essentially unattended; one driver sleeping inside while the other leaves to get food doesn't count as "attended" in my mind.
I don’t see how it matters to the client though; the security measures could be entirely non-existent, but that’s why the insurance policy exists. The quality of security should only impact the rate at which insurance has to be paid out, so it’s already the insurance companies’ problem and really only impacts them
Put another way, it was a business decision.