They use nickel plating in an electrolytic bath and that goes everywere, including the outside. Nickel does not adhere well on its own and one can use chromium as a thin adhesion layer before putting on the nickel, hence the term "advanced bi-layer".

High performance engines only use a thicker chromium layer. This is true for car engines (and some airplane engines) from OS too, but why they insist on making sub-pair airplane engines I have no idea. At least they don't claim to be making ABC engines anymore, like they use to.

I agree though that this engine looks like it has been severely overheated, so more a user error than a fabrication error.