It’s a classic definition. Soft/hard science fiction has two meanings either the topic is focused on hard sciences (physics) vs soft sciences (sociology) or “It can also refer to science fiction which prioritizes human emotions over scientific accuracy or plausibility.[1]”

So it’s not universal but is an accepted definition that any deviation from the possible or probable (for example, including faster-than-light travel or paranormal powers) to be a mark of "softness."

https://en.wikipedia.org/wiki/Soft_science_fiction

Popular science fiction is generally extremely soft, but occasionally you get stuff like The Cold Equations where the plot is driven by real world constraints. Even then it included FTL so a purest would call it soft.