As with any fan theory, there are some holes in this. For such a small guy, Ant-Man is a particularly big wrench here. Scott Lang escaped the Decimation because he was in the Quantum Realm at the time. We know that he eventually finds his way out and in a much different world. Lang is seen approaching the Avengers compound in the Endgame trailer, with Captain America and Black Widow watching him on a monitor. If Cap and Widow are in the Dead World, then how is Scott there, too?
Article source: https://www.cinemablend.com/news/2466118/what-if-the-dusted-avengers-are-actually-the-ones-who-survived-the-snap