Throw in hits backboard

[Hoosier]

During game last night, I am administering the throw in on the baseline in the backcourt. Defense is pressing. The throw in hits the side edge of the backboard and is caught by a teammate. I allow the play to go on, but "C" comes across and says it hit the backboard. I asked him which side and he said it didn't matter on a throw in. I thought that all edges of the backboard were inbounds. If had hit the back of the backboard I would have whistled the violation immediately. Am I wrong on my interpretation of this? I typically follow the principle that if the ball deflects any way but backwards, it did not hit the back of the backboard.