I'd like to extend what's been happening at that moment from my perspective.
To my mind it was more directly connected to what people were saying nearby when they found Taiga's student ID and the picture with Taiga and Kitamura in it while commenting on that she really seems to like him (Kitamura).
My interpretation is that Minori already realized her feelings for Ryuuji long before and therefor desperately wanted to believe that Taiga might genuinely like Kitamura, not Ryuuji, after all (although deep inside she knew otherwise) , hence surpressing her feelings of guilt towards Taiga.
Not even Ami could know more than that at that point of time but she heard what was being said by the others and she knew about Minori's feelings and her reaction to the talk so with her comment she pretty much hit the bullseye.
ps: yes, I just rewatched the series, so my impression is rather fresh
