They've been voiced by those individuals, as opinions and examples, not as proven facts or even supported theories. Who said something doesn't make it more or less valid, it matters whether what they said is backed up by evidence. To say that something is more likely to be correct just because it's Sagan or even Einstein who said it, is an "Appeal to Authority". Physicists didn't just accept General Relativity because of who proposed it, they tested it themselves.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort