I think there is a miscommunication here: There's no 'if' about "should people, even fully vaccinated people mask up" - yes they should because they _might_ be

I think there is a miscommunication here: There's no 'if' about "should people, even fully vaccinated people mask up" - yes they should because they _might_ be carrying the COVID-19 virus, albeit at a lower chance of infection than unvaccinated people. And if they are infected. which is less likely but does happen, they will still carry and spread COVID. That's where the "if" goes.