I think it's interesting that the quote says Manwë had not revealed it to them, not the Valar had not. We know that Manwë had the deepest understanding of the Music of the Ainur of all Valar. The Elves learned much from each of them, maybe it was understood as Manwë's 'job' to teach the Elves about subtleties of the Music. In this case there was no necessity to reach a decision for the Valar not to tell them, and it was up to Manwë what to reveal and what not to, just like it was probably up to Aulë what to reveal about, say, metallurgy.
I agree that it was a misjudgement of him to withhold this information. The reason for the Elves' uneasiness about the Secondborn was that Melkor told them. Who knows what exactly he said? I don't think there would have been reason for mistrust had Manwë taught them at times.
So why didn't he? It is said Manwë was wholly good, so he must have had a good reason and no crooked second thought. Maybe it's just not so easy to teach about the Music and to understand what is taught? It's been a design of an extremely high subtlety after all and even the Valar don't know everything about it. This might sound silly, but: maybe ten thousand years weren't enough for Manwë to get to the Third Theme?
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