I agree that the ansible isn’t hard sci-fi, it’s not defined rigorously IIRC.

However I don’t think hard sci-fi has ever been strictly limited to known physics. It just requires consistency with it.

Greg Egan is one of the OGs of hard sci-fi, and a lot of his work is borderline psychedelic in how far he takes speculative-yet-consistent physics.

There are plenty of physically consistent “FTL” mechanisms in the family of “bend spacetime” that don’t violate c and have not been disproven.

By that construction, it seems like "hard sci-fi" would allow little to no creativity to push the envelope unless the author is also a theoretical physicist.

there is almost no sci-fi with at least some invention and speculation that bends known physics and/or engineering, either implicitly or explicitly. novels that seem very “hard” often ignore effects that would ruin the plot, and make up physics. it might be plausible and compatible with known physical laws, but it’s speculation plain and simple.