Imagine I get the Windows source code and rename the variables by adding a "314" after each varaible, after each function name and rebuild Windows, in your definition this is remixing and fair ?
Where you like it or not this is an undecided area both legally and morally. Pretending it's clear cut is either disingenuous or delusional.
My simple example is to show that is not as simple as "the AI earned from N devs GPL code and now it can spit new original code without ZERO concerns", we know how this stuff works and that it can spit out the exact training input in some cases.
So IMO a judge should ask the question "from how many people you need to steal, mix the input to be sure the output is actually original".
And about the thing "if I read someone code it is not stealing" , hyumans are different and even for humans it is not allowed to read the code of your competitor and then write new code using that knowledge.
The word "remixing" here is useful because it will fit any conclusion the reader prefers.
Arguably even in your reductive example, the result would be non-infringing. Or not. Which conclusion you reach is exactly the topic under debate. Isn't this textbook question begging?