I've been seeing this theory around, and while it does make sense, I feel like an issue could be the fact that it would mean that not only Garp, but Big Mom and Kaido all know about Imu, as well as some other Rocks members. I feel like it would take away from the mystery if Puddings Mom could have told her about Imu before beditme lol.
And yeah, I agree that God Valley was probably destroyed by an ancient weapon, though I think some survivors made have made a town similar to it/based on the culture, unless Oda retcons the Shanks getting alcohol from his hometown in the west blue