Because the process is different. You and any computer agent are allowed to learn the functional, non-copyrightable elements of fast inverse sqrt. When you need that functionality, you can write code that implements your understanding of those non-copyrightable elements and gain copyright over the resulting creative expression.

What you can't do is copy all of the creative expression in the original (such as comments) without complying with the terms of the license. Moreover, reproducing the magic constants is a strong indication that your process didn't independently derive your code because the constants used in the original are unique and non-optimal.