Epic spells are generally described as generally weaker—or at least more cumbersome and difficult to make more powerful—than true 10th-level and higher spells. See Lost Empires of Faerûn for a discussion of that. Since some of the darklords had 10th-level and higher spells prior to Die, Vecna, Die!, and those couldn’t do it, epic spells can’t do it either.
Whether or not the rules in the Epic Level Handbook reflect this is kind of inconsequential, because we expect the Demiplane of Dread to be exceptional in this regard, and don’t expect Epic Level Handbook to cover that exception. Many, many magics—including teleportation—just work differently in Ravenloft.
As it is, only two effects have ever allowed someone to leave Ravenloft without the Dark Powers’ apparent blessing: Azalin’s Grand Conjunction, and Vecna’s stunt during Die, Vecna, Die!. Both of these things relied upon unique circumstances that will never repeat: the Grand Conjunction on a prophecy that has already come to pass, and Vecna’s strategy on abusing loopholes in the natures of Ravenloft and Sigil that have since been mended. Any other route out of Ravenloft will necessarily be similarly unique and involved—casting an epic spell may be involved, but it will certainly take more than just the one spell.
Indeed, the Epic Level Handbook claims that epic magic was part of Vecna’s scheme, to allow him to capture Iuz’s divinity. The epic spell didn’t have remotely enough power to free Vecna from Ravenloft directly, though—it had enough power (under the circumstances) to make Vecna a god, and then from there it was the power of Ravenloft itself that expelled Vecna. He engineered the situation, but ultimately the real force involved came from the Demiplane of Dread itself (and thus—probably?—from the Dark Powers).