I think they're mixing up cause and correlation. My theory is flying animals evolved shorter guts to save weight, and it's the shorter guts that cause a lack in

I think they're mixing up cause and correlation. My theory is flying animals evolved shorter guts to save weight, and it's the shorter guts that cause a lack in biome diversity. The weight of the gut bacteria is irrelevant, it's just a side-effect of gut reduction which probably constitutes a larger % of bodyweight.