This may be well known, but I'm posting it because I didn't know: "very-hard" science fiction in this context means extremely plausible science fiction, as opposed to extremely speculative science fiction. The author explains how these fantastic things exist in a way which is realistic.
Originally, I thought it meant "very hard to understand" i.e. very technically complicated
Originally, I thought it meant "very hard to understand" i.e. very technically complicated