It wouldn't even need to be human insulin, since insulin is not and has
never been a bacterial protein; they simply don't need it.
However, that might not be conclusive. Afterall, poppies don't need neural
transmitter mimics like morphine or codeine, yet they produce them. So it
is conceivable that a bacterial species could evolve a protein identical in
function to insulin, even though it don't need it, and it might even have
the same basic structure as insulin.
But by chance alone it would never be identical in amino acid sequence to
human insulin, so your original point still stands.
Kevin L. O'Brien