Well, "my" theory is also pretty wrong if it doesn't allow for the human civilisation that built Stonehenge.
A theory that is wrong will sometimes predict correct things by accident. That's unavoidable, but we a) have statistical methods for modelling how often things go right by chance, and b) have a philosophy of science in which theories are forever tentatively held: your aliens theory might beat any theory which says Stonehenge is impossible, but will soon be replaced by a theory which explains Stonehenge more economically.
A theory that is wrong will sometimes predict correct things by accident. That's unavoidable, but we a) have statistical methods for modelling how often things go right by chance, and b) have a philosophy of science in which theories are forever tentatively held: your aliens theory might beat any theory which says Stonehenge is impossible, but will soon be replaced by a theory which explains Stonehenge more economically.