Zaydun is definite because it is a proper name, yet it has tanwīn of tamakkun because it is declinable. Hence tanwīn does not universally signify indefiniteness.
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A Definite Name with Tanwīn
Tanwīn: Five Types, Their Function, and Omission
Zaydun is definite because it is a proper name, yet it has tanwīn of tamakkun because it is declinable. Hence tanwīn does not universally signify indefiniteness.