If I had to guess, you don't need to use it.
A = randn(500);
timeit(@() eig(A))
ans =
0.0800828164545
B = A + A';
timeit(@() eig(B))
ans =
0.0064739664545
Do you see that eig ALREADY runs roughly 12 times faster on a symmetric matrix of the same size?
So I would bet the MATLAB eig checks for symmetry, and then uses an appropriate call to LAPACK. (The above test was performed using R2022b, update 1.) So your old source was just that - old, and out of date as long as you are using a current MATLAB release.
