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.
The US Library of Congress had the following back in 2008. The last paragraph seems to be the important part for defining Science Fiction, but I included the two above it for extra context into their view:
"Throughout this statement particular attention is given to science fiction (although it is, in fact, a sub-genre of fantasy) because it dominates the genre of fantasy in terms of the total number of titles published. It will be the general rule, therefore, throughout this statement to speak of "fantasy and science fiction" together and on equal terms. Unless otherwise specified, however, the fantasy provisions below apply equally to all of the other sub-genres of fantasy.
Fantasy includes the sub-genres of science fiction, horror and adaptations of traditional myths. The distinguished writer, Arthur C. Clarke, has stated that "any sufficiently advanced technology is undistinguishable from magic." (Omni, April 1980, p. 87.). This view is borne out by the fact that the distinctions between science fiction and the various other sub-genres of fantasy are indeed blurred at times and usually artificial. In fact, many authors in the genre frequently cross these artificial barriers in mid-work or in mid-career. Publishers, furthermore, often confuse these sub-genre identifications even further by failing to differentiate among them. Publishers do, however, frequently identify books in these various sub-genres with tags which usually appear on the spine or cover of the individual books stating that they are specifically fantasy, horror, science fiction, etc. These tags may be very useful in identifying materials whose precise classification is doubtful or subject to various interpretations. Although difficult to define with precision, fantasy usually requires a willing suspension of disbelief. Works in its various sub-genres often 1) adapt, rework, or provide an alternate telling of a myth or folktale; 2) involve an alternate reality or alternate universe; 3) rely on a displacement of time or space; or 4) make use of elements of the horrific, supernatural, paranormal, or the occult.
2. Science fiction
In addition to sharing any or all of the general characteristics listed above for fantasy, science fiction usually 1) is speculative in nature; 2) assumes change as a given; 3) projects a story-line into the future or into an alternative reality or history; 4) explores a problem in technology, culture, philosophy, etc. beyond its current state; and 5) presents an atmosphere of scientific credibility regardless of the reality. Not all science fiction 1) takes place in the future; 2) involves space travel; 3) describes technology beyond current reality; or 4) deals with alien cultures. However, these elements are common in this sub-genre and uncommon outside it."
The full document is at: https://www.loc.gov/acq/devpol/scific.pdf