It's a very common theory but not explicit. It all makes perfect sense though

It's easily his best film. Everyone shits on it because it was another "twist" while completely ignoring the excellent dialog, acting, character building, and theme. At its heart, it isn't a story about an invasion. It's the story of a grieving family leaning on each other for support after a tragic loss.

Gibson's character had been outwardly the strong, moral man of faith at the center of his family and the community. His wife's death pulled him into deep despair that had left him a broken man. He no longer had hope for the future and had stopped providing hope and comfort to his family and community.

Throughout the film, you see not only his grief over the loss of his wife, but everyone else's grief over losing him as a fixture of compassion, comfort, and grace. The story is about his faith and hope being restored. The events that lead to that involve hostile invaders, but they're just a mechanism to tell the story of grief and healing.

There's a scene with Phoenix and Gibson whispering quietly for like 10 minutes about belief and fate with no action - just great acting and dialog. There's a scene where he breaks down crying over his family and how cruel he's become due to his fear and hopelessness while simultaneously eating a meal.

And in the end, many of the aspects of his normal life that contributed to his grief, annoyance, and fear were key to his family's survival. He accepted what life had thrown at him, and was able to heal.