...that has no proof, alongside "Rumia is Yukari's daughter", "The unnamed book youkai is Rinnosuke and Keine's lovechild", "herp derp X is Y's Z" etc.
God, this is why I'm starting to hate the TH fanbase, no harm intended for you guys if you believe the theories, just saying things out loud.
Ohai take it easy, they are fooling around they aren't discussing theories seriously, it's just that Flandre saying Rumia's catch phrase made them feel like jocking about Flandre = Rumia.
I'm all for fan speculation, seeing as 95% of touhou continuity is mostly fanon - it's when people start to get aggressively defensive over their pet theories that it starts bothering me.