It is true that if two people happen to independently create the same thing, they each have their own copyright.
It is also true that in all the cases that I know about where that has occurred the courts have taken a very, very, very close look at the situation and taken extensive evidence to convince the court that there really wasn't any copying. It was anything but a "get out of jail free" card; it in fact was difficult and expensive, in proportion to the size of the works under question, to prove to the court's satisfaction that the two things really were independent. Moreover, in all the cases I know about, they weren't actually identical, just, really really close.
No rational court could possibly ever come to that conclusion if someone claimed a line-by-line copy of gcc was written by them, they must have independently come up with it. The probably of that is one out of ten to the "doesn't even remotely fit in this universe so forget about it". The bar to overcoming that is simply impossibly high, unlike two songs that happen to have similar harmonies and melodies, given the exponentially more constrained space of "simple song" as compared to a compiler suite.
All of this is moot for the purposes of LLM, because it's almost certain that the LLMs were trained on the code base, and therefore is "tainted". You can't do this with humans either. Clean room design requires separate people for the spec/implementation.
That's the "but their case would still fail if the second author could show that their work was independent, no matter how improbable" part of the post you're responding to.
One out of ten to the power of "forget about it" is not improbable, it's impossible.
I know it's a popular misconception that "impossible" = a strict, statistical, mathematical 0, but if you try to use that in real life it turns out to be pretty useless. It also tends to bother people that there isn't a bright shining line between "possible" and "impossible" like there is between "0 and strictly not 0", but all you can really do is deal with it. Where ever the line is, this is literally millions of orders of magnitude on the wrong side of it. Not a factor of millions, a factor of ten to the millions. It's not possible to "accidentally" duplicate a work of that size.
It sounds to me like you're responding to a different argument than they're actually making and reading intent into it that isn't written into it.