Children are psychologically self-absorbed, not in any condemnable way but just as a legit and functional developmental stage. As IJCWTBK clearly illustrates, Simba has no shadow of a doubt that the universe revolves around him, so why doubt that your uncle knows something like that about you? You assume that the center of everyone's awareness is your place in the universe.
Is why children think it's something they do wrong when parents divorce at a young age--everything revolves around them so it must be something they did wrong. Simba just gets exceptionally slammed by that dynamic, and this just proves that that's the stage he's at that he DOESN'T question that Scar knows everything about what he's up to. Children assume that's just how it is.