He worked within the conventions of his time. See earlier contribution mentioning Dryden. A work of forgery based in part on existing tradition and/or some written sources does not become "more genuine" than if it were lies from end to end, though I suppose it might bring some of this pre-existent material to wider notice. There's a conflation, or confusion, being made among several aspects of "the Ossianic controversy", in a way that's never done -as far as I'm aware, though things might have changed in recent years - when re-use of historical material, adapted, elaborated, distorted and accompanied by sheer invention occurs in works by Walter Scott, Byron, Wagner or even Defoe. For whatever reason, MacPherson is being judged by standards quite different from other writers, and this has been the case for nearly three centuries.