I think the critical factor that makes sci-fi hard isn't known physics, exactly, but early on establishing a known set of rules and letting them play out as they will. The narrative has to respect and adjust to those rules, not vice versa.
This ends up driving most hard sci fi to current understanding of physics, with maybe one tweak. If you diverge more from that, it's difficult or impossible to establish the rules and their implications early on, and so choices about them later end up serving the narrative.
Though, a certain irony: hard sci-fi doesn't seem to offer much predictive insight about the future compared to other subgenres. The 20th century sci-fi genre that seems to offer the most insight into our current moment is probably the one with the wildest imaginings: cyberpunk.
This ends up driving most hard sci fi to current understanding of physics, with maybe one tweak. If you diverge more from that, it's difficult or impossible to establish the rules and their implications early on, and so choices about them later end up serving the narrative.
Though, a certain irony: hard sci-fi doesn't seem to offer much predictive insight about the future compared to other subgenres. The 20th century sci-fi genre that seems to offer the most insight into our current moment is probably the one with the wildest imaginings: cyberpunk.