Yeah, things got real crazy when Saturn and Jupiter got into a 2:1 resonance and forced Uranus and Neptune to switch positions.
A bigger problem is that Earth had a very toxic Venus-like atmosphere for the first half a billion-years. And yes, the oceans were at, like 300°, but the oceans didn’t boil away due to the increased air-pressure.
To my mind, the crux here is whether or not there are other places on Athas that are capable of supporting as many people as the Tablelands. If the answer is yes, then, I think the thematically likeliest explanation as to why none of the SM or Borys seem to have much of anything to do with that place (think of the resource extraction possible, at the very least) is that it’s got its own Dragon or an equivalent. Ultima Thule and “here be dragons/lions” and all that.