If the scifi is unexplainable, then it cannot be "hard" scifi.
Science can be intuitive (as a kind of protoscience that at least coheres with what is known to be scientifically true). And scifi can speculate about things that may or may not be possible, such as time travel or FTL (faster than light travel).
But "hard" means things that are scientific facts are made part of a scenario for a thought experiment.
Well I didn't say things shouldn't be explainable, just that it isn't worth it to focus on it.
The film Arrival for instance is as hard a scifi film as it gets. It doesn't waste time trying to be a textbook on its speculative science. Peter Watts novel Blindsight, which has vampires, only gives a general explanation of why they exist, but doesn't waste time trying to justify their existence by breaking down the exact evolutionary process that developed them.
Its "explained", but not really. Only enough to be reasonably plausible.
And in actual fantasy, the line for reasonably plausible is a lot more forgiving.
Ive described it before, but even in 5e as it exists (and even its harder mundaneity ancestors), just the fact that an ostensible normal human can fight a dragon in melee combat establishes a baseline well above real human capability, even if its mechanically unsatisfying.
That dragons exist at all frames fantasy towards a forgiving plausibility line.
The movie Reign of Fire, interestingly, goes the other direction though and arguably approaches hard scifi more than fantasy. The humans are genuinely human and dragons are as devastating to them as you'd expect, and the dragons were designed to be about the most realistic take on dragons that existed when it came out; a feat it accomplishes so well its Dragons have basically become the ancestors of all depictions of dragons since in film, tv, and video games.