From what I can tell, “Hard Science Fiction” is also known as Just Science Fiction, and generally it means to readers books about boy science stuff not icky girl stuff like anthropology or archaeology or political science.
If if you can’t tell from my tone, I am saying there’s not really such a thing as “hard science fiction” it doesn’t exist. The head editor of Analog Magazine lists Flowers for Algernon as his preferred prime example of hard science fiction, and no one seems particularly concerned about speed of light or pure science accuracy, in that story.
I've written hard sci-fi, I guess, with one novel and many short stories, including stuff in Analog, so I am not an absolute authority, but I know a little bit.
Hard science fiction is just just science fiction being marketed to an audience that includes scientists and people who play scientists on TV, so to speak.
(I have written scientifically accurate stories based on new research and been raked over the coals by “scientist” reviewers, so maybe include citations if that bothers you. Science is a wild and woolly thing and everyone is very certain they are right.)