I think that's a bit misguided to be honest. TRON doesn't fit in EPCOT either, it's just that the aesthetic is similar to Test Track. Moreover, Guardians is a vastly superior attraction and EPCOT was (and still is) in need of attractions that draw attendance.
To the greater point being made by
@UNCgolf - they could have told this story or a similar story without IP. I don't think the connection is the worst in the park, but it's absolutely a leap relative to EPCOT from 1989.
The primary objection I have with the story is the presence of the Celestial as the reason for kicking off this ride. They could tell the same story with a humanoid alien stealing the Cosmic Generator. That being said, the attraction is anchored by a talking tree and raccoon like creature so we need to be able to accept some level of silliness.
I knew definitively that they didn't care when Tom Fitzgerald said at the D23 Expo that EPCOT is where the real becomes fantastic AND the fantasy becomes real.