It justifies it by precedens. Aster means star - astronauts have been called people travelling to stars for a long time, even though interstellar travel is not possible for us yet, so naming their means of transport Starship is not out of line. It just follows a kind of funny naming tradition.
I’m not sure how the context of the word “astronaut” justifies implying interstellar travel that you aren’t capable of.
It justifies it by precedens. Aster means star - astronauts have been called people travelling to stars for a long time, even though interstellar travel is not possible for us yet, so naming their means of transport Starship is not out of line. It just follows a kind of funny naming tradition.