Good point. It would have been better if I had been using something more like, "Placing the story in the future is a sufficient, but not necessary, indicia that it is science fiction." After all, not only do you have novels like Frankenstein, you also have a number of alternate histories that take place in an imagined past that are usually considered science fiction (and, weirdly, often involve Nazis and/or dirigibles, but that's another issue).
Of course, the problems with defining the term itself have long been noted, and it is usually better to be over-inclusive with the umbrella term than under-inclusive.