Before the release of the Ezra Miller-led film, Gunn had clarified that it would "resets many things, not all things," and "some characters" would stay after the reboot. Co-CEO Peter Safran also said that the upcoming reboot would "take some sort of explanation," assuming that The Flash would be it.
However, the explanation of time travel and the film's ending doesn't add up to a reset of the DCEU that would naturally lead to a DCU reboot.
How Time Travel Works in The Flash
When Miller's Barry Allen traveled to the past to undo his mother's death in The Flash, he mistakenly believed that he would have created a branching timeline, similar to Back to the Future or Avengers: Endgame:
"I traveled back in time from here to here, yet, somehow, everything's all changed back here, like when you were born."
Of course, using a spaghetti metaphor, Michael Keaton's Bruce Wayne corrected him while also referring to potentially both films' interpretations of time travel.
"Well, time isn't linear, right? At some point, you probably saw a movie that told you if you went back and changed the past, you create a kind of branched timeline, right?"
Instead, Wayne explained a form of time travel atypical of sci-fi films: the idea that changing the future, therefore, changes the past and vice versa, more commonly known as retrocausality:
"When you go back and change the past, you create a fulcrum. You put yourself on a whole 'nother strand of spaghetti. New future. New past. It's retrocausal. It goes both ways"
Yes, it's a unique and fun form of time travel that's not commonly used in films and makes sense in a vacuum, but it doesn't truly allow The Flash to reboot the DCEU like Gunn or Safran suggested.
The Ending Isn't Really a Reset
It's blatantly obvious that this method of rewriting history would have fit into the original plans for the DCEU set in place by Walter Hamada, but those plans became obsolete when Gunn and Safran came in.
The Flash originally ended with Keaton's Bruce and Sasha Calle's Supergirl being alive again, establishing that Barry had once again changed things. However, that (and many other) endings were scrapped and replaced with a Geroge Clooney cameo.
Gunn already confirmed that Clooney won't be the new Batman in the DCU, which means Miller's Barry didn't even end up in Gunn's new universe. It's understandable why it was basically impossible for the ending to truly set up the DCU, but it still feels misleading for him to say it would.
He's the co-CEO of DC Studios and has to sell these movies to audiences, even if it means exaggerating certain details. Fortunately, two words from Keaton's Bruce and a shoddy post-credits scene could give him and Safran an out.
James Gunn and Peter Safran's Out
After Keaton's Bruce explained to Barry how time travel actually worked, he continued his spaghetti metaphor to describe the Multiverse:
"What you did was change the future and you changed the past. If a person is stupid enough to mess with time, what you eventually end up with is this: the Multiverse"
But what's important is how, according to him, some universes ran "almost parallel" while others could be "wildly divergent:".
"Some strains run almost parallel, there will be inevitable intersections, and others that are just wildly divergent..."
Those two words, "wildly divergent," are the out Gunn and Safran need for the latter's "explanation" of the reboot not to be completely untrue. After all, the new DCU could very well be one of those "wildly divergent" universes Bruce mentioned, which is also why some actors will still stick around.
No. No, no, no. 'Cause what I'm trying to explain, as simply as I possibly can, I promise — they were all Batman. They were all Bruce Wayne. But each of them is a completely different person!
How Barry references "all" the Bruce Waynes seems inappropriate if he's only talking about Affleck and Keaton and he's still in the Clooney universe. So, it's possible that in this post-credit scene, Barry tried to return to his universe again, only to end up in the DCU.
Regardless, considering how The Flash has been an unmitigated box office disaster, it's doubtful that audiences will ever see Miller's Barry again. The film isn't the big reboot that was promised, but it had nuggets of exposition that gave audiences a semblance of an explanation.
The Flash is currently playing in theaters.