We are told in TFA that she imagines an island while trying to sleep, and in TLJ she says that she’s seen the island ‘in dreams’. You’re right that a dream sequence wouldn’t outright contradict anything, but the only prior time in which this type of sequence has been done in Episodic Star Wars has been Anakin and his Force visions. I think there’s a strong assumption in Star Wars that such a sequence would be Force related even if it’s not explicit, and that’s the sort of assumption that I want to stay away from at the beginning of the movie anyway.