Author of the post here, I'll try to elaborate.

My primary association with the word "fantastic" is simply "great" or "very good", as in "you did a fantastic job!". It's this meaning of the word that I found oddly disconnected from the word "imaginary". How do you get from something meaning "not real" to something meaning "very good"?

But the word "unreal" helped me make the leap, since that's a word I would use to describe something I thought was "very cool" or "well done".