> The Martian not qualify as Sci-Fi, hard Sci-Fi at that?
I didn't say that The Martian "does not qualify as Sci-Fi" at all. You are misreading.
I said "traces" of sci-fi, i.e. a non-zero amount of it; as it's all very grounded in actual fact, despite being set in the near future.
In what way is "Sending humans to mars" a "big idea"? - it's talked about and planned for a lot. It's not a surprise if it happens. It's not a reality yet, that's the "trace" of extrapolation, of Sci-Fi.
Submarines were not actual sci-fi in 1870 https://www.history.com/news/9-groundbreaking-early-submarin...
I don't think that the "hard" part is relevant. A book set on an oil rig today, or even on the ISS, with similar "hard" engineering challenges would have a high "hard science" content, but that wouldn't make it sci-Fi. See: techno-thriller (1).
Because sci-fi is not just "fiction about science and engineering ... that already exists". It is extrapolation into the unknown. You have a point that "The Martian" does extrapolate. Counterpoint: Not by much.
1) https://en.wikipedia.org/wiki/Techno-thriller