Sort of, but more just mocking a small inconsistency in the default presentation of the items. In the DMG is the magic item Adamantine Armor, but that doesn't necessarily mean all armor made with adamantine is magical, that's just a specific example they've provided. Likewise in Xanathar's Guide we got rules for weapons made with adamantine. They are not inherently magical but that doesn't mean you couldn't have a magic weapon made of adamantine, that's just not a specific example they provide directly in the rules on adamantine weapons. You can view more discussion on it here https://rpg.stackexchange.com/questions/92657/are-weapons-and-armor-made-of-mithral-or-adamantine-considered-magical and here https://www.sageadvice.eu/why-is-an-adamantine-armor-a-magical-item-while-a-adamantine-weapon-is-just-one-made-with-a-diferent-material/ as well as reddit, xitter, etc.

tl;dr The "default" presentation of Adamantine Armor in the DMG is a magic item, the "default" presentation of adamantine weapons in Xanathar's is they're nonmagical.