DY: I think that Minhagim are very strong. I’m saying Kitniyos is Assur MiTa’am Minhag, not a real D’Rabannan. It makes perfect sense to use the word “Assur” in that context, but it is not a real “Isser” per se. Does that make sense?
And there are some real Nafka Minos here. For example, I know of major Poskim who are Mattir Kitniyos for those with (real) gluten allergies/celiac disease, because limiting them to only potatoes (and now quinoa) all of Pesach is just too much. That would never happen with something we treat as a D’Rabannan, but by a Minhag there is a tiny bit more wiggle room.