Science Fiction doesn't have to be fantasy, it can be speculative. But if your setting or plot relies on something we know to be scientifically untrue, and you don't put some effort into explaining why it somehow works in your setting, it's fantasy and not speculative.

Someone like Asimov never considered his books to be fantasy and that he could just insert whatever he wanted with no justification. In fact, he never considered sci-fi to be a genre, he always argued it was a setting and that his most famous stories were detective stories in a sci-fi setting. But detective stories don't work if your world isn't grounded in something real. Otherwise the reader can't reasonably build their own theory or deduce the answer because it's based on what the author thought was cool and not what logically connects.

The appeal of something like The Expanse just falls apart if you introduce a FTL engine just because it makes for a more dramatic story moment somewhere in the plot unless there is some serious justification as to why the author didn't just break all the rules of their world (which is supposed to be our world, but in the future).