Sorry but it is true. I am using an objective definition which is also the literary definition (which is why I quoted it right out of a literature textbook). Trying to define it the way you are doing is subjective - we wind up with it being a matter of opinion whether it is science fiction or not.
I posted the literal definition.
This:
"In other words, the fact that the setting is an interstellar spaceship means genre = science fiction."
...is not true.