Yes.
What people call “finger” depends on the language they speak natively. In English, thumbs are not considered fingers. In many other languages they are.
What @DeaconElie means is “every thumb is a digit, but not every digit is a thumb”. Some other examples would be “every cat is an animal, but not every animal is a cat”, “every whisky is a drink, but not every drink is a whisky”, and “every cheater is a rule-breaker, but not every rule-breaker is a cheater”.
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