This is just wrong. First of all, IBS != IBD. Second of all, IBD doctors will flat out recommend high fiber Mediterranean style diets for anybody who is not in an active flair.
Low FODMAP is also recommended for IBD and is quite reputable although not a silver bullet. I agree with the above poster that Fiber if anything is an old hat approach which can make things worse as easily as they make things better. Low fiber diets are used ALL THE TIME in the treatment of ibs/Ibd, as are high fiber diets.
I am rather convinced quite a few peopl have IBS/IBD in significant part due to their high fiber diet which tend to be high in FODMAPs if not meticulously designed and inflame the intestines with solid residue. People blindly recommending fiber as a panacea, and I’m not saying it doesn’t have a place in the treatment of IBS/IBD, but just blindly throwing fiber at the problem as if it’s always going to help us legitimately going to land people in hospital.
A big advantage to the low FODMAP diet and why it’s taken off is that it can treat constipation and diarrhea without having to adopt a high fibre diet.