Well biological structures can be replicated mechanically. So at risk of being pedantic, that's not strictly true.

I view "artificial" as just a term for conscious, purpose construction, whereas "natural" does not involve any conscious intelligence, as in "naturally occurring in nature." But even that is a spectrum, I guess, with genetic manipulation, BCIs, all sorts of intermediate possibilities.

I'd call a constructed, purely biological brain "artificial" because, even if it was grown naturally, it was built with conscious intent.