> You mean, like those silly science fiction stories where FTL travel is possible? Or time travel?

FTL or time travel are not necessarily unscientific - we know that relativity permits CTCs to exist, they're something that can be explored rigorously and scientifically. Even for stories that adopt decidedly unscientific handwave versions, one dropped stitch won't necessarily unravel the whole garment, especially when it's not the focus - if you're telling a story about life on Omicron Persei 3, how one gets to Omicron Persei 3 may well be beside the point. But yes if a story is full of things like that, even for things that are the focus of the story, then it's not science fiction.