Even knowing the April Fool's origins I can't help but interpret it as a Discordian style satire of both copyright for trying to claim exclusivity and copyleft for going from undermining the previous to its own social contract. I am more than willing to admit that the idea may be from viewer preconceptions and overthinking as opposed to any inherent message beyond the inherently silly instructions.
Copyleft puts the traditional concept of copyright on its head. "Copyright" limits "who can make copies." If I hold copyright on a work, only I can make copies (or only I can decide who else can make copies.)
Copyleft uses copyright law to enforce a different scheme, a social contract. The license, which can only be assigned by the holder of copyright on a work, insists that licensees (you, if you have access to my work which I've put under an appropriate license) must also allow copying by anyone, even when you modify my work.
I don't think this is an evolution, (the parent to your comment doesn't actually claim so); I think it was the entire original intent by Stallman. What has evolved is the terms of the GPL license, attempting to keep up with changes in technology.
This sort of reminds me of those copy pastes people would put on their Facebook walls claiming that they had no right to their photos. Are facts even license-able the same way a formula or code would be...
