This statement is also messy because there are plenty of offerings in Tomorrowland that fit into Epcot (which is a common talking point among Disney Adults). At the end of the day, this comment doesn't come from any source of appreciation for the park, but from nostalgia. EPCOT has always had science fiction in its dna. Whether it be Dreamfinder and Figment, Captain EO, those classic outfits Mickey and the gang used to wear. What separates the reality of a man and his dragon searching for imagination to inspire people and a man and his raccoon going back to the big bang?
A non-Guardians big bang coaster would also be science fiction since you'd have to explain how you got there with time travel. Not to mention Body Wars and shrinking down? Cranium Command and being in a fictionalized version of the human mind. As much as Horizons was about "future" living, it was very much a science fiction ride. Jules Vern is there!!!
It is okay to be nostalgic for what was there when you were a kid. But it is disingenuous to bring up this specific issue for Guardians because sci fi has always been part of EPCOT's identity.