NCL's cancel policy used to insinuate that if you canceled due to a positive covid test, you'd get an FCC, however if you were denied boarding because of a positive covid test could get a refund in lieu of an FCC. That link was available last week, it no longer goes to that policy anymore: https://www.ncl.com/refund-and-cancellation-policy-covid-19
If that is still the case, it seems you need to force NCL to deny you boarding...