Ha ha ha ha that's fucking awesome the Led Zep one isn't fake, I recorded it myself and tried.
However the sound does require some doctoring in order to make some syllables more pronunciated than others and to cut out nonsense syllables. There's also easily more than one interpretation of exactly what the backwards singing sounds like.
There's little more to this than novelty, though; it's not unreasonable to assume that in the countless millions of hours of popular music produced you could play some of it backwards to find distinguishable messages. And it's not hard to make them sound sinister when you're playing something backwards.