Second-hand refers to whenever there's a middleman or medium (such as TV or audio) relaying what happened to you. Like if Dante's Inferno was written this century, the book would be considered a second-hand account of Hell, or if the Emperor of Japan said he had the regalia of the three Shinto Kami as proof of his divinity (often with people unsuccessfully asking if he has proof of having proof or proof that it is proof), it would be a second-hand account of his authority. We would be going by their word, and anyone writing anything that disputes it would make it a "he said she said" spat. People get angry at me often if they show me a recording of someone saying something and I say it's not 100% definitive because it's not unchallengeable, as opposed to someone taking me to the action, directing me there. If, for some reason, the moon landings were to be challenged in a court of law (no, time has shown Mythbusters is not a court of law), these would be the inquiries/protocol taken. And suppose things came up such as "we recorded over the original footage" or "we brought these souvenirs back but cannot verify that a human brought them back" or "Buzz Aldrin punched someone who questioned him about the landings", do you think maybe such quirks would raise a few eyebrows? The Soviets we cannot speak for, especially since both Cold War blocs were putting words in each others' mouths all the time, in fact we know the capability of the Soviets to keep a good secret was enough that they could create whole top-secret cities that civilians still don't know about, which means we cannot say the US government couldn't have faked something without it leaking out, that's the whole point of being able to keep any classified documents.