they never even referred to here being beaten and killed in the first one. ohp! kh2! she's alive?! i killed her a year ago!! in Halloween Town, at least they were like, "oh, Oogie Boogie is dead" "Jack killed him" "I'll revive him!"
They must've conveniently forgot the fact that Ursula is supposed to be dead in order to bring back Atlantica so they can show what happened in the real LM. I mean, without Ursula, the story of LM won't be there at all.
But yeah, the trio should've remembered that Ursula was defeated. Or maybe that particular memory of them was wiped.
It just ocurred to me that the reason why we were made to go back through the worlds twice was just so that they can show the a part of the real story of that world. >,<