If you haven't finished the book yet I don't want to spoil it for you, but that quote from Elrond is simply a foreshadowing of things to come at the end of the story. As far as Frodo keeping it a secret, it dealt only with the Ringbearers so there was no reason to tell the others.
If you've already finished the book and are still confused let me know and I'll try to explain it.
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"Dic, hospes, Spartae, nos te hic vidisse iacentes dum sanctis patriae legibus obsequimur."
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