Your post is a bit confusing. Of course anyone can get covid. The vaccine simply has reduced severity of the disease. Won't discuss the rule as not allowed, but will state there were MANY cases reported on board Summit in August with many more ill passengers who did not report. The passengers who were ill and did not quarantine simply spread the virus to others.
The rule is what it is. You either comply or don't cruise. Absolutely no difference between testing on a B2B than my flying to Iceland and testing there.