Urborg, Tomb of Yawgmoth

Land cards not on the battlefield aren't Swamps while Urborg is on the battlefield.

Urborg, Tomb of Yawgmoth isn't a Swamp while it's not on the battlefield.

If an effect such as that of Magus of the Moon causes Urborg to lose its abilities by setting it to a basic land type not in addition to its other types, it won't turn lands into Swamps, no matter in what order those effects started to apply.