Thor can and has very much damaged considerably harder metal than true Adamantium with his strikes, such as Captain America's shield and the Destroyer armour, and he defeated his uncle Cul, who completely shattered Captain America's shield.
Kurt Busiek, despite being an excellent writer, had an extremely lowballed view of Thor's full power, explicitly placing him far below the Silver Surfer, Binary, or the severely depowered Post-Crisis Superman, and as such depicted him having to go all-out to damage even secondary adamantium during his prominent Ultron storyline, but this was a typical Marvel Comics inconsistency.