The insured has a duty to disclose everything that is or would be material to the insurer, even if that information is known only to the insured.
Fraud is entirely separate.
I wrote a super long comment below, but on a more fundamental level... how?
Especially for something like travel insurance. Anyone the average consumer gets to talk to about a travel insurance policy is going to be some call center jockey being paid on commission.
In fact, my insurance agent sold me a home insurance add-on for personal electronics. Their pricing didn't take into account the cost of the covered electronics. Replaced some very expensive hardware after (inevitable) failures. When I purchased the policy, I sent my agent and email pointing out that I has some super expensive electronics, expected short shelf life, and was pretty damn certain that the policy had positive expected value.
The agent straight up asked why I wouldn't buy the policy in that case!
That product was discontinued, of course. The insurance company tried to claw back one of their payments until I showed them my emails with their agent. But the fact that they even tried is... telling. And I'm just lucky they didn't press the issue, because I probably wouldn't have bothered going to court over a few hundred dollars in wasted payments.
There's a fundamental problem with trying to apply this sort of legal doctrine when most consumers are unsophisticated and don't have access to the same data as the insurer. The fact that we only ever get to talk to sales people compounds the problem.