Theory in the works: Edelgard Stans argue about characters, backstories and story information solely through the lens that they also experience and attach themselves to Edelgard.
Edelgard does not change or develop in a meaningful way in the story of 3H, no matter what route is chosen. This is an intentional writing decision as it relates to her archetype and role in the story.
But the ramifications of that are her stans now viewing every character in Fire Emblem Three Houses the exact same way.
So even though Dimitri, Claude, Rhea and almost every other character changes through Byleth's guidance through the academy and the war effort, Edelgard Stans will nonetheless argue through the assumption that they're still the same when Byleth first meets them (as well as sometimes sprinkle in the assumptions/views that Edelgard herself has of the characters she has an opinion on).
So Dimitri is still wishy-washy (they usually just cut the middleman and call him a centrist) and secretly deranged, violent and even racist and a cop.
Claude still wants to murder Rhea/wants her gone and believes the church needs to be abolished in order to open the borders to other countries.
Rhea is a secretive, manipulative megalomaniac and tyrant who must be taken down for Fodlan to be free.
Felix still hates his father and Dimitri and wants to be free of his house, Sylvain and Ingrid hate their obligations to being Crest bearers, Lysithea hates Crests first and foremost, Dorothea wants nobles to vanish, Caspar wants to dedicate himself to Edelgard, Dedue and Cyril see themselves as no more than tools of their white masters, etc., etc.
Because Edelgard Stans' favorite character doesn't change, they subconsciously assume the same must be for the rest of the cast.
That's why arguing with them is a lost cause, because while anyone else is operating with a cast that, by the end of any non CF route-actually, any route really considering CF's explicit negative character development-changes and becomes someone new, Edelgard Stans are operating with a cast that hasn't moved past first impressions and initial character flaws, coupled with their own biases for how these characters should talk and act.
There's no winning with people who are applying a, shall I say, "Chapter 1 Thesis to an Epilogue Debate." When they're convinced they know the correct and best interpretation of a character, as evidenced by how they attached themselves to a character intentionally written to not develop or change, that should tell you more than anything else could reveal.
