That situation was similar to the bug where you could get Shadow Ball on a Mewtwo.
It's just an accidental bug nothing more. It's not intentional, Niantic doesn't want you getting legacy moves outside their alotted time frame. Yeah sure, there is a small chance that they could mess up again but don't depend on it.
The best, and most confident answer I can give you is to just wait for December 2019.
