Exactly.
If the Shadowlands existed since the first mortal was born then, the Emerald Dream existed since the first capable dreamer was born.
Knowing so little about the Emerald Dream is a good reason to have an Emerald Dream expansion. If they managed to pull off 6 different Shadowlands zones out of, basically, nothing then, they can do the same with the Emerald Dream. These would be:
"Dreams can have varying natures, such as being frightening, exciting, magical, melancholic, adventurous, or sexual."
https://en.m.wikipedia.org/wiki/Dream