The theory, which comes to us from Reddit, has to do with Ant-Man’s seemingly inevitable journey through time in Avengers: Endgame. The mid-credits scene of Ant-Man and The Wasp showed Paul Rudd’s Scott Lang stranded in the Quantum Realm, as Hank, Janet, and Hope faded to dust as a result of Thanos’ snap (aka The Decimation). Just moments before, Janet warns Ant-Man about time vortexes, so smart money indicates that the hero will end up traveling through time before eventually appearing on The Avengers’ CCTV footage.
Article source: https://www.cinemablend.com/news/2467140/new-avengers-endgame-theory-brings-hank-pym-into-the-story