I think something can still be hard sci-fi and be inaccurate based on newer science. The original War of the Worlds might qualify in that it brings in astronomical concepts to the story and they serve as background and plot elements. It might be possible to derive exactly when the story takes place from astronomical clues left in the story even though it is never explicitly stated. At least from memory, nothing about the laws of physics as they were known at the time is changed for the story. The interplanetary launches as if they were from a gun are implausible, but we don't really know that it is a gun - the characters assume it but it isn't explicitly stated that the acceleration of the projectile occurs over a timeframe as if it were a gun.
Star Wars is probably at the other end of the spectrum - it makes no attempt to adhere to physical law as understood at the time and outright misleads the viewer as to the nature of the universe for the sake of "rule of cool", with the density of the asteroid field(s) as one of the egregious examples, but carelessly placing worlds and moons near each other for the sake of visuals and so on.
Classic Traveller is at least somewhat harder sci-fi than Star Wars: planetary systems are of realistic scale. The vector movement rules are in retrospect a dramatic simplification but at least had respected conservation of momentum and treating space as a vacuum, it wasn't possible for small winged fighters to turn or bank against the vacuum of space and very large build ups of relative velocity are possible in Traveller. Though most official Traveller materials presented interstellar travel in 2D it would have been possible to build a 3D map, though additional non-canon rules would need to be added. For example, the direction of a misjump can't be just in the direction of one of the hex edges selected by d6, +Z and -Z rules would need to be added and aren't.
Psionics gives the whole thing a very non-hard-sci-fi vibe though. Later versions of Traveller seem to try to abandon vector movement to just vague range-bands which seems like a move away from harder scifi, but T5 also does include rules for relativistic time dilation. Ship acceleration and performance in general is based on the maneuver drive potential and volume of the ship. You can't dump your cargo to gain additional acceleration in CT, yes in some of the materials "mass" is said instead of "volume" for some of the ship components (which turned out to be an error).
If there were a -10 to +10 scale of "pure fantasy" (-10) to "science fantasy" (0) to hard science fiction (+10), AD&D might be a -10, Star Wars a -8, 2300 AD a +2 and CT a 0, or so.
That would either be a) our disparate notions or b) something the game designers never explained in sufficient detail (or have explained multiple times in seemingly contradictory ways).
That's the challenge here.
Agreed. And it's a tough one. So no wonder we instead jump to better-understood "yardsticks" like "science" to try to make sense of things. But I'm suggesting that impulse is, at its core, a sort of distraction from the genuine challenge at hand.
YMMV, of course.