this is bordering on pedantry but aren't there classifications by different regulatory and medical institutions of what fibers are and aren't? as far as I'm aware, a starch isn't a fiber unless a special exemption is carved out for it and only if that starch meets certain criteria for how it behaves
looking into RS's in general, it seems like there is a special carve-out after a lot of industry lobbying for very specific forms of RS's to be classified as fibers and only if they have physiological effects similar to the classic definition of fiber: https://www.fda.gov/media/113663/download
for practical reasons, I suppose I'm just not understanding why you don't eat more 'classical' fibers. psyllium husk, for eg, which is cheap and effective and has been utilized as a digestive aid in Chinese traditional medicine for centuries (one of the few to survive rigorous empirical evaluation, naturally)