"[...] revealed no signs of cometary activity after it slingshotted past the Sun on Sept. 9 at a blistering speed of 196,000 miles per hour (87.3 kilometers per second)."
That's greater than the escape velocity of the Solar System. Also: It has no "orbit" (elliptical or circular path around the Sun) - rather, its trajectory is hyperbolic, as ground-based observations indicated.
Regards,
It is a calculation based on assumptions. They assumed that there was no prior event that caused it’s current trajectory. Direct evidence would have been Actually seeing it in another galaxy, but as of right now that isn’t exactly possible.
Cheers,