It could be that their story involves something that they think others won't understand or believe. For example, if it's a very strange or personal spiritual experience. Also, perhaps they've been through something that is so unique and difficult to put into words that they don't want to even attempt to tell it, as they are worried about not being able to convey it accurately and having people misinterpret it.
Often, it indicates that the story contains elements that are either very personal, like a deep secret, or it has the potential to cause some sort of upheaval. Maybe it's a story about a past mistake that they are ashamed of and they don't want others to know. Or it could be a story related to family secrets that are better left untold. They feel that it's almost a sin or a very bad idea to share it, so they use this expression.
Well, God Forbid is purely fictional. It's the imagination of the creator and not drawn from actual happenings. There are no real-life equivalents or inspirations for this story.
I'm not sure specifically which 'god forbid real story' you're referring to. It could be a very personal or specific account that only the person using that term knows about. Maybe it's a story that involves something difficult or tragic that one hopes never actually happened.
God Forbid on Hulu is most likely not a true story. Usually, such shows are created for entertainment and may incorporate fictional plots and characters.
One example could be a story of a family who lost their home in a natural disaster like a tsunami. The force of the water just swept away their entire livelihood, and they were left with nothing. It's a 'god forbid real story' because no one wants to experience such a tragic event in real life.