Also, Epcot is a different park, in that it's not meant to transport you to a different time and place.
The park itself is the place, the world's fair.
So, having Guardians take place in the actual park works, in that sense
(even though the ride is obviously in the wrong park)
It's the same conceit for the Awesome Mix show at the American Pavilion. The Xandarians are there to share their culture and music since World Showcase is a cultural exchange.
Then Star Lord comes out and shows them the true American imperialist spirit by making them assume American music and culture.