That's not true at all. Like I said, copyleft relies completely on copyright. It was not at all created in opposition to copyright. It was created in opposition to proprietary (i.e., "unfree") software. Here, from the Wikipedia article on Copyleft (emphasis mine):

The concept of copyleft was described in Richard Stallman's GNU Manifesto in 1985, where he wrote:[6]

GNU is not in the public domain. Everyone will be permitted to modify and redistribute GNU, but no distributor will be allowed to restrict its further redistribution. That is to say, proprietary modifications will not be allowed. I want to make sure that all versions of GNU remain free.

This is impossible to do without copyright existing. Like I mentioned before, without copyright laws, companies can take any code they wish, with any license, and use it in their proprietary software without sharing their modifications.

Another reference directly from the FSF: "Copyleft is a tool that works within copyright law" (emphasis mine)