Algorithms for generic mathematical operations such as the dot product or matrix multiplication are often trivial to deduce, though optimizer vectorized versions perhaps less so. Most helper functions are unoriginal enough that no reasonable copyright law would protect them, which is also the case for (too) many cases of patented code.
The copyright question does ignore the code license question, though. If a complicated algorithm like FISR is not original enough the what protects any boring old operating system code? What stands in the way of publicly hosting Microsoft's leaked sources, as clearly the code is all quite trivial? There is very little in an operating system that other operating system developers haven't thought of or would reasonably have come up with had they been constrained to the same restrictions.
The variable names are one thing, though they could be chosen much more descriptively. However, the system also output the comment "// what the fuck?" which is not only terribly nondescriptive, it's also something that the system couldn't have come up with if it would have learned from code in any practical form.
The suit you linked is about the difference between information and creativity. However, the case surrounds a data set, something simply factual, rather than a composed piece of information such as code or a book. Code listed on Github is not similar to the listings in a phone book. If they were, all software copyright, proprietary or otherwise, goes down the drain. I think that's impractical to say the least.
FISR could have been patented (and be now in the public domain anyway), but only it's specific implementation in DOOM is covered by copyright.
Also, your argument follows a composition fallacy: emergent properties exist, and thus you cannot simply say that because each individual piece of a whole is trivial, the whole is trivial. Heck, software pretty much by definition goes against that. For relevant precedent, there is no shortage of information that becomes classified when in aggregate. Knowing where a certain piece of infrastructure is isn't likely classified, but knowing where all the strategically important pieces of infrastructure are certainly is.
Which is why the question isn't whether the users of Copilot are infringing someone's GPL (they'd likely have a solid defense based on the individual piece not being sufficient to hold copyright protection), it's whether Copilot itself constitutes a derivative work of its input data, which it consumed as whole (copyrighted) works.
A human would probably try to defend against a copyright infringement suit over that by arguing something like the following.
There isn't sufficient creative expression in fast inverse square root (FISR) to be copyrightable. There is plenty of creativity in that thing, but it is in things that are not copyrightable such as the underlying mathematics that it is using. Copyright covers expression of ideas, not use of ideas (that's patents) or the ideas themselves.
The expression in FISR that they probably are copying from is pretty much all just in choosing the names of variables, and most implementations I've seen just use pretty normal names that follow normal naming conventions that people use when they aren't putting any thought into naming their variables.
That level of expression is arguably not creative enough to support copyright, at least in the US after Feist Publications, Inc., v. Rural Telephone Service Co., 499 U.S. 340 (1991) [1].
(I'm assuming that the human didn't do anything stupid, like reproduce the comments too).
[1] https://en.wikipedia.org/wiki/Feist_Publications,_Inc.,_v._R....