"Dreams are windows into the lives of our multiversal selves"They literally establish all dreams are universes. They even make a joke about it when Wong talks about the dream where he runs naked from a clown and America Chavez says "it's somewhere out there".
"So that recurring nightmare where I'm running naked from a clown" "somewhere out there it's real"
But what's also important to note is that America Chavez states "that was [Defender Strange's] theory" and that doesn't necessarily mean *every* single dream is in fact another reality just that dreams theoretically are windows into other realities.