You're missing the point.
Richard Stallman *****HATED***** copyright. Hated it with a passion. He found the very concept repugnant. So, he wrote the GPL to essentially say "do whatever you want with this, just don't say you invented it from scratch and don't prevent anyone else from doing the same." The GPL is essentially about removing copyright restrictions, and preventing someone else from re-implementing them back onto the same body of work.
So, while copyright law does in fact make the GPL enforceable, the whole pint of the GPL is to use copyright law to remove copyright. Hence why it's often called copyleft - compared to a normal copyright, it's kinda the logical opposite.
And if we're getting into poop-flinging on "logic 101" I recommend studying what logic actually is. Formal Logic; Informal Logic (aka Natural Language Logic); Symbolic Logic; Mathematical Logic... there's several different types of "logic." This discussion revolved primarily around the informal variety, which your parent post used correctly.