So because you insist that EVERYONE is exposed then they should just lie about it? The difference is while we all likely may be exposed in our everyday life, no one - not even you - can just assume that everyone is exposed. The question isn't do you "think" you were exposed, it is "were you" exposed. The point of the questionnaire is to determine if you know you have exposure, as the OP hubby does. Answering honestly is the right thing to do for the "greater" good of everyone on the ship, isn't that the point?
I too am sure that most people will not answer yes to this question even if they know they were exposed because they are thinking only of themselves and missing out on their vacation. I get it, that's a tough thing to happen. But you know what - it also isn't right of you to assume the worst of someone that is in reality doing the right thing. You immediately jump on them that they must be trying to get out of their cruise why else would someone do something so foolish? In reality the OP's husband is just a man of the highest integrity that likely answers to a higher power in his heart. I commend him. The world is coming to what it is because most people think only of themselves... its the me, me, me syndrome.
If the OP's hubby has his convictions then I say answer yes. Carnival may or may not deny boarding, they may just be required to go through additional testing which may delay their boarding a little. Again big deal, the ship isn't leaving without them (again the me, me, me syndrome - got to beat everyone on the ship). And worse case, if they can't go, at least he can sleep at night knowing he did the right thing and didn't sell out over a few bobbles.