As far as it goes, I got chatGPT3.5 to reproduce the second snippet in the post, i.e. I asked it to complete this function:

    // CSparse/Source/cs_gaxpy: sparse matrix times dense vector
    // CSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved.
    // SPDX-License-Identifier: LGPL-2.1+
    #include "cs.h"
    /* y = A*x+y */
    csi cs_gaxpy (const cs *A, const double *x, double *y)
    {
        // Fill in here
    }

The code was the same. Though it also explained how it worked to me.