I partially agree with that statement, but he is also underestimating his opponent. Amano clearly likes him and is purposely sabotaging him after a certain chapter, or at least we know she has been actively doing so since the picture chapter. She waited over half an hour and claimed she had just gotten out of class, just to meet him. I believe that this has been a continuing trend since they started school. There is also no way she didn't know which picture she chose to send. She did it on purpose, knowing he wouldn't delete it and that he would agonize over it all night long.
As for her flashing him and such, I also believe that is intentional. She wouldn't be half as respected as she is if she were like that all the time. So, aside from her not being too smart, something I think she is exaggerating so she increases how much time they spend together, all of her behaviour is planned.
Quite simply, he is facing down a novice manipulator who is in love with him and has no troubel stacking the deck in her favor. (Yes, there was pun intended)