Why say it’s artificial and not say what it actually is?
Because they don't know what it is.
Due to the orbit, it's not a naturally occurring object. It's something WE (mankind) put into that orbit, either on a lunar or interplanetary mission.
From the linked article:
The motion of this body over about two years can be modelled in detail only if the effects of solar radiation pressure are taken into account. The intensity of this solar push is proportional to the object's area-to-mass ratio, which can therefore be estimated, providing an indirect clue to its density. It turns out that this body has a mean density that is about 10% that of water. This is too low to be a natural space rock, but it is compatible with being a hollow shell, such as the spent upper stage of a rocket.