My point about the FTL thing is that this question is in the realm of science fiction. Sci-fi authors can come up with whatever physics they want, and in real life we are so far from being able to do it that we can’t tell how it’d look like. So I wouldn’t rule out that it’d be based on some physical principles that allow a non-weaponizable spaceships.
Regarding the comparison to cars - I agree that it all depends on definition, but while there is some merit to the philosophy that “there are no wrong definitions” - bad definitions are certainly a thing. And a definition of “weapon” that includes regular cars is a bad one, because it misses out the important distinction between regular cars and armored vehicles with mounted guns.