Okay so yes Silverstone is fake and is not canon. They only mention Sliverstar because they are the main character of KtL. Since Sliverstar didn't exists the true canon is that Littlebird killed Firestar. Ik it's confusing but that's why in the wiki it says, *fiction*
I think it's up to the player to decide what version is canon. For me, the reason Robinfur would say that Littlebird killed Firewing is because it was a very common misconception that she did kill Firewing instead of Silverstone.
