Spoiler
In Against All Things Ending, maybe two or more times Linden points out her lack of direct knowledge of the Land as Covenant first encountered it (I clearly remember that she notes this about Morinmoss redeeming "the covenant").
Subconscious inspiration can unite people's imaginations outside their knowing intent. We know Linden and Covenant are similar in personality in crucial ways. Doesn't it stand to reason that if both were randomly met by a cryptic homeless man who made them feel mysteriously challenged, and dealing with the concepts underwriting the cult, and on and on, that they could pretty much share a dream? Only it could not be just like the dream Covenant had about the Land ten years earlier. So it's massively deformed.
A slippery internal logic allows this explanation to make only as much sense as its alternative, though. I see good evidence for the parallel-universe thesis, also. I'm just trying to come up with a way for the Land to really be imaginary (for Covenant and Linden as well as for readers) despite its appearance of chronological order.