The latest episode of Marvel's Agents of SHIELD had a gentle dig at Avengers: Endgame. Over the last year, time travel has become a central part of the Marvel Cinematic Universe, both on the big and small screen. The approaches were slightly different, but both were essentially a variant on the Multiverse theory.
Avengers: Endgame's time travel has proven to be pretty controversial, not least because the writers and directors disagree about the basic model of temporal mechanics the MCU employs. Part of the problem appears to be that Marvel changed their time travel "rules" partway through production. The Ancient One's all-important explanation, which served as an info-dump to help viewers understand the film, was actually reshot late in the day.
Agents of SHIELD season 6, episode 9, "Collision (Part 2)," seems to gently poke fun at all this confusion. At the end of the episode, a delighted Deke is reunited with his "grandfather" Fitz, and can't help excitedly chattering about his theories of time travel. "Oh and I don’t want to gloat," he notes, "because technically the argument was with the other you, but you being here, you prove that my multiverse theory was right."
In context, that single line of dialogue cuts to the heart of Agents of SHIELD's model of time travel. Where Avengers: Endgame was confusing and inconsistent, Agents of SHIELD's time travel rules were surprisingly well-formulated. The SHIELD team was trapped in a time loop, and the fate of the planet itself rested upon whether or not they could break it. Like Bruce Banner in Avengers: Endgame, Fitz insisted that the laws of quantum mechanics meant that it was impossible to break the time loop. As a result, he didn't believe SHIELD could prevent Earth from being destroyed. In contrast, Deke argued for the Multiverse theory, that actions create branches in time, alternate realities where anything is possible. Deke was right; SHIELD stopped Graviton and saved the planet, thereby creating a new Agents of SHIELD timeline.
But it seems that, for the last year, Deke has vaguely wondered whether or not his theory was actually correct. He'd presumably wondered whether SHIELD hadn't identified the real reason Earth was destroyed; given Deke lived on the West Coast, it's easy to imagine him reacting in terror to every slight tremor, expecting the entire planet to shatter beneath his feet. But there's absolutely no way to reconcile Fitz's return from space to Deke's timeline. The Multiverse theory is therefore proved to Deke's satisfaction.
The dialogue may fit perfectly with Agents of SHIELD, but it's hard not to see it as a subtle dig at Marvel Studios' model of time travel as well. Avengers: Endgame explored the Multiverse as well, but in a far more inconsistent way; the Ancient One seemed to insist only the removal of Infinity Stones created an alternate reality, but that fits uncomfortably with Captain America's fate, where he traveled to the past and lived out his days with Peggy Carter. The Russo brothers have insisted he created an alternate timeline, and traveled back in order to hand over his shield to the MCU's Sam Wilson, which directly contradicts the Ancient One's explanation. Deke's throwaway comment is a nice way of reminding viewers that, unlike Avengers: Endgame, Marvel Television formulated a model of temporal mechanics and stuck to it. Precious few science-fiction series pull that off, and it's quite right that Agents of SHIELD should celebrate it.