I'm not sure how that verse is related to flat-earth but as far as God stopping the sun and moon, or the earth, it seems to me that the creator of this universe can change the rules and do what he likes with the sun, moon, and earth. It's also possible that what happened is that nothing actually stopped bit the earth and sun and moon aligned in such away that it appeared as though the sun and moon stopped, for a brief time. We see extremely rare and unusual astrological events from time to time. As I said I am not an expert, perhaps someone more educated can shed some light on that verse for us. Thanks for commenting!