Are magic items excluded from benefiting from an adamantium coating via Xanathar’s rules?

Hey @JeremyECrawford, dumb question but you know how D&D nerds love to pick apart rules. Are magic items excluded from benefiting from an adamantium coating via XGE’s rules? TIA

— Chris Chapman (@GammaViking) December 17, 2018

The rule doesn't require the weapon to be nonmagical.

— Jeremy Crawford (@JeremyECrawford) December 17, 2018

I think the confusion is on whether you can add adamantine to an existing magic weapon. Nothing in the rule prevents that.

— Jeremy Crawford (@JeremyECrawford) December 17, 2018