Doesn't that mean you assumed it was similar size, speed, or distance as another plane? If you really have no idea what it is, I can't think of any way you could distinguish a large fast distant object from a small nearby slow one. It's too far away for our binocular depth perception to measure distance.

I hope I don't sound too critical. I wondered if the thing in the article's story was some small object ejected from the pilot's own plane and he overtook it as it slowed down so it appeared to be moving backwards. Since you had a similar experience, I wondered if there was some other information that helped you judge it.