> I think the critical factor that makes sci-fi hard isn't known physics, exactly, but early on establishing a known set of rules and letting them play out as they will

No that's just competent writing. Hard sci-fi is very much about real science.

Clockwork Rocket by Greg Egan is specifically about physics which does not exist here.

"Hell is the Absence of God" by Ted Chiang deals with a world in which god, heaven and hell are unambiguously real.

Both are definitely "hard sci fi"