Yeah Carnival sure isn’t advertising any such restrictive policy, because they know a lot of people won’t book if negative test is required and their money would be lost if the test is positive. On their Covid update page they specifically say there is a FCC if you test positive within 10 days of cruise.
As for insurance, the last 2 years have been a nightmare for people who bought travel insurance only to discover their pandemic covered issues were less than stellar. But besides that it is moot- if you book a cruise with a written policy allowing for FCC in case of positive test then that’s the policy you should expect.