But we'd be able to look at his clone code and see it's different, with different algorithms, etc. We could do a compare and see if there are any parts that were copied. It's certainly possible to clone GNU grep without copying any code and I don't think it would fail any copyright claims just because the GNU grep code is in the wild.
If that was the case, the moment any code is written under the GPL, it could never be reimplemented with a different license.
So instead of a human cloner, I use AI. Sure, the AI has access to the GPL code - every intelligence on the planet does. But does that mean that it's impossible to reimplement an idea? I don't think so.
Just because something is trivial enough to copy does not mean it was trivial to conceive of and codify. Mens rea really does matter when we are talking about defrauding intellectual property holders and stealing their opportunity.
But then how can the FSF reimplement AT&T utilities? The FSF didn't invent grep. They wrote a new version of it from scratch under a different license.