I'm not missing the point. Clause 22 says anybody with symptoms of any virus will be denied boarding, without compensation. P&O are simply applying this condition when anybody who says they have such a symptom is denied boarding. P&O are not claiming that the person has Covid, or anything else. So the only way to contest the decision is to show the clause is invalid. Is the clause arbitrary and grossly unfair? Almost certainly, but that doesn't necesarily make it invalid.
Like so much else in life, people have to make their own minds up about their appetite for risk, and decide if they want to sail with this clause in the contract. Of course, if they are led to believe that insurance will cover them if the clause is invoked, and that belief is unfounded, that's a different matter.