I played a lot with imaginary friends and in imaginative worlds as a child and I did it until I was much older than most NT kids. In fact I still make up imaginative worlds and stories in my head. Though my trauma therapist thinks that it was a way for me to cope with/escape from trauma, but I've always wondered if there was a little more to it than that.
I think it's important to note that something can be a trait or indicative of a condition without it being problematic or maladaptive. For instance I used to be really into collecting business cards. (I still have my collection, but I'm just not nearly as active in seeking them out as I used to be.) In the report from my neuropsych evaluation when I was diagnosed with Aspergers the psychologist specifically noted that I had asked for a business card from everyone in the office and that I had explained that I had a collection of over 400 business cards. She included this in her report because it shows that I have intense special interests that are unusual which is a characteristic of Aspergers. But my collection was just out of the ordinary, not problematic or maladaptive. In fact it could be argued that it was helpful because I interacted with more people.
My abnormal psych professor explained that there is a difference between a behavior or trait being out of the ordinary and it being maladaptive. The two sort of lie on the opposite ends of a continuum and the point where something goes from different to problematic can be murky.
That being said, I could perhaps see how an extreme preoccupation with imaginary friends or an imaginary world can become a problem if the person hardly even interacted with other people because of it or became distracted so they miss important information from their environment. But again, just because someone does something more than what is ordinarily seen, doesn't automatically make it maladaptive or problematic.
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"Curiosity killed the cat." Well, I'm still alive, so I guess that means I'm not a cat.