«إما أن يكون زيد في البحر أو لا يغرق»
Either Zayd is in the sea, or he does not drown.
The book uses this as an example of exclusion of joint falsehood. Its intended point is that both clauses can be true: someone may be at sea without drowning.
The book treats their joint failure as excluded under the intended supposition. Outside that restricted setting, a person can drown in a river; the example is therefore not a universal physical law. We are studying the logical form under its stated or understood assumptions.