Your interview might have made them acutely aware that your extension exists and they may have either got a copy and looked their site's products to find issues or at least take care of the "low hanging fruit" examples you used.
So what occurred might have been as you say. And I suspect you didn't move forward on the interview process BECAUSE of your extension (i.e. they didn't want to muddy the waters in case Amazon's legal department wants to send you a C&D letter) but I don't think they needed to interview you to get intelligence on your extension (since any engineer worth their salt at Amazon could have told them the same thing, just by downloading a copy and reading).
It sucks that Amazon didn't see the value of your innovations and exploit them to make their site better. But their loss I guess...