Well, clearly I am not going to be able to explain this to MMTM, but as noted above, it's a relatively harmless obsession and might get you to try interesting new foods.
Lets try to break this down another way.
When Roberfroid published his theory of "prebiotics," he defined it as
"A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health."
This is an abstract, hypothetical (and unoriginal) idea. Not an actual substance. (BTW, The term "prebiotic" had already been used for more than a hundred years to describe organic chemistry that occurs in the absence of living organisms. I don't know why he couldn't choose a different word.)
To a dietitian, this is pretty much the definition of "dietary fiber." Naturally, any dietitian will recommend that you consume plenty of dietary fiber. (Unless you have certain bowel disorders.) It's exactly the same thing.
To the supplement industry, it means oligosaccharides. Either fructan or galactan. Why? Because they are already produced in industrial quantities. Not because there is any evidence that they are particularly beneficial. It's easy to package them in pill or powder form and sell them as "prebiotic." (Alternatively, exactly the same stuff is sold as sweeteners and thickeners.)
But you don't need to seek out sources with industrial concentrations of fructan or galactan (e.g. the lists that you find in Wikipedia.) All fruits and vegetables have plenty of diverse oligosaccharides. About the only ones that really would not fit the above definitions are starches. There isn't much evidence that one is better than another. But most of them have really not been studied in this context. (Or even analyzed carefully - it's not easy to do. Much easier to just call them all "soluble fiber.")
BTW, the paper referred to in the OP found no detectable effect from fructan pills (what you might get from chicory) and did claim an effect from galactan pills (what you might get from acacia sap, but is mostly produced from milk). But if you look up research on the galactan pills, what surfaces is that the European Food Safety Authority has twice rejected
any health claims made for this stuff. The paper did not demonstrate any effect on microorganisms.