Using the logic you've already employed, one could conclude that the entire anthology of philosophical texts also have zero bearing on reality because "they're all made up".
Societal and philosophical questions don't work like that, ethics are driven by society and the view of society changes over time with actions like, as an example, eating beef on Fridays being incredibly taboo among christian societies a few hundred years ago (and to this day in some areas) to the point where taking an action like that in public would likely call into question your ability to act ethically.
Ethics is a highly subjective field - if you try and revive geocentric solar system models with a paper that essentially amounts to "I said so" you'll be dismissed since there are theories with more evidence out there. The process of allegorization though is a more subjective and iterative process where two people can honestly state very different theories and have wide support from large groups - including potentially having mutual supporters.
The allegory that comes from Sci-fi isn't a statement of fact, it's an opinion on danger and while objective evidence is always preferred it doesn't mean that subjective evidence is meaningless. The process of going back and forth forever is quite valuable, if it isn't being done with "No, you're stupids" then each participant will be further developing their theory to account for weaknesses or vagaries exposed by the other participant and that process produces a better understanding in both parties and more refined theories to be presented to spectators.
The question of whether it's morally correct to commit murder is still open - there are some really compelling arguments in favor of not murdering people and some of those even arise purely out of self-interest[1] but if I say "Bob murdered Jim" there are all sorts of qualifiers and conditions that can take that from being pretty ethically repugnant (say infanticide) to being generally seen as justified (maybe, ongoing debate yo) at least in the US (say a genuinely necessary and unavoidable act of self-defense).
The problems fantasy is really good at digging into are ethical ones and, physics based fantasy books are super boring. If your sci-fi book was about how the earth would be different if G was actually 1.367×10−10 m3⋅kg−1⋅s−2 you would essentially have a book about how some mechanics of movement are affects and why planes are so expensive to fly - you could build an interesting plot, but the allegorical value would be essentially nil... well that's what I think, lesse is some sci-fi writer comes along and writes a super impactful and interesting novel about an earth that is essentially the same as ours but with slightly less than two times as much gravity.
1. Which tends to be the strongest moral guidance IMO, but I'm jaded :shrug: