If I said “I ate 7 cookies, all chocolate chip” surely it wouldn’t be ambiguous? :)

Yeah, it was a bit ambiguous. They should have picked better wording, one of the following would do to clarify.

> ... 25 initial signatories — all of them Fields Medallists —

OR

> ... 25 initial signatories — all of the living Fields Medallists —

The first one is what they meant.