Thranduil wasn't the only one who made a statement about the mail being made for an elf-prince.
From The Hobbit-
Quote:
"Mr. Baggins!" he (Thorin) cried. "Here is the first payment of your reward! Cast off your old coat and put on this!"
With that he put on Bilbo a small coat of mail, wrought for some young elf-prince long ago.
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So Thranduil's statement does not stand alone. The narration backs him up.
But do we know for certain that Legolas had no brothers?