I can't understand this explanation for the following.

"Example (29): British citizens can only vote in the Brexit referendum.

(29) however means not only that being a British citizen entitles one to vote in the Brexit referendum, but also that being a British citizen only entitles one to vote in the Brexit referendum but not in any other elections. So the subject matter is not British citizens but the elections in which a British citizen can vote. Let E = being an election in which a British citizen can vote, R = being the Brexit referendum, the variable x then denotes an election, rather than a person.

(29’) ∀x[(Ex → Rx) & (Rx → Ex)], which is equivalent to ∀x(Ex ↔ Rx)"

The proposition just explains that British people can't vote anywhere else but the Brexit referendum. But that doesn't necessarily mean that if there is a Brexit Referendum it is necessarily going to be British citizens who can only vote(and thus only an election in which a British citizen can vote) in which case why is If R then E correct?

Using another example

"Scientists are only clever". "If Scientists then clever" would be true but it wouldn't be "If Clever then scientist"