Servius, Commentary on Aeneid 1.601:
“TYNDARIDIS: [Helen is so called] although she is the daughter of Jupiter and not of Tyndareus; but we say ‘Tyndaridis’ of Helen just as we call Hercules ‘Amphitryoniades.’ For Pollux and Helen were born of Jupiter and Leda, while Castor was born of Tyndareus, who alone was mortal. For this reason, Vergil says (6.121) ‘if Pollux redeems his brother with alternate death,’ since he shared his own immortality with his brother.
A reckoning of time, however, suggests that Helen was immortal. It is agreed that Helen’s brothers were with the Argonauts: the sons of the Argonauts then fought against Thebes, and the sons of the warriors at Thebes waged the wars against Troy. Therefore, had Helen not been immortal, she would undoubtedly not have been able to last for so many generations. We read, however, that she was earlier taken by Theseus and entrusted to Proteus in Egypt.”