You’re being absurdly literal - 25% of US citizens within the context of a study does NOT linguistically suggest it means 25% of total living US people. The fact that no one else in this thread shares your interpretation shows you completely lack the ability to parse basic comprehension of the English language without taking the absolute most literal path, which given the nuances of the English language, is usually the wrong interpretation.

Do you think any statement which starts “x% of Americans…” Suggests they surveyed every single American alive?

Maybe they should specify “alive” though, because not telling you they’re not including past Americans who have died is dishonest.