You are correct: by the standard definition, the Roadster is not a celestial object. It was just loose terminology in the tweet. When you look up the Roadster

You are correct: by the standard definition, the Roadster is not a celestial object. It was just loose terminology in the tweet. When you look up the Roadster on the HORIZONS database it doesn't call it a celestial object. To see the details, first click the link in the tweet: https://ssd.jpl.nasa.gov/horizons.cgi?CGISESSID=c3cbd47fbf603007d1b627107c28962f&s_body=1#top https://ssd.jpl.nasa.gov/horizons.cgi?CGISESSID=c3cbd47fbf60... Then click the Cancel button at the bottom of the page. It should now show SpaceX Roadster for the Target Body, and a Generate Ephemeris button at the bottom. Click that button for details.