True, but it pretends to have its own science and technology. Dune may be the greatest sci-fi series but it’s also not really based on our current science and technology.
Discussion
Science fiction needs to be at least plausible given the right technology.
Fantasy can be anything. Any magic.
Blade Runner is scfi
Termintor is scif
Preditor is scifi
Hmm. Just looked this question up and apparently George Lucas agrees with you. I guess the Force would make it more fantasy.
What makes Star Wars fantasy and Dune sci fi?
Imho, if there is a plausible scientific component, however far fetched, and no magic, mysticism, voodoo, pseudoscience, ghosts, etc. it is scifi.
Most people just lump them together. Scifi/Fantasy section.