My only thoughts (and this is completely grasping at straws) is whether they thought these questions or discussion didn't show enough interest. Given what you have said, I still think this would be pretty harsh even if it was the case though.
Ahhh thank you at least it's not just me thinking it's harsh haha! I'm thinking maybe I didn't do anything per se wrong in the interview, but they just didn't think I was a good fit/they thought there were simply just better candidates, and wanted to just try to give me some form of constructive feedback? Onwards and upwards anyway!