"The relatively rapid drift of the signal is also puzzling for other reasons. A planet would have to be rotating nearly 40 times faster than Earth to have produced the observed drift; a transmitter on Earth would produce a signal with a drift of about 1.5 hertz per second. "

Maybe it's an object in orbit. Maybe it's a time beacon and they want a changing signal- say it's on a elliptical orbit. Maybe it's powered by orbital energy- and it's crossing of magnetic lines of a dense planet or moon that is powering it's signal. The object in orbit could massive and thereby be capable of sending a powerful signal and have long lifetime. Or could natural object in an elliptical orbit [made of some crystals or iron?] which converting magnetic fields into radio signal.
If it's a time beacon it could have no message or very simple message, but even it there is no intentional message but it's "Alienmade" it could still possible tell us something about it's makers.