I really didn't want to come across as pedantic, which is why I tried to keep it light and say I thought his mimicry was cute. And I love stuff like this! We need more 6th graders like this. I would have mad chilled with him and his app club.
That said, I do believe that the way we use language frames the world for other people. So I try to keep things in perspective. It's not that this isn't wonderful, it's that there're so many wonderful things that I prefer specificity of compliment to a general word ("great") that may suggest this is some sort of pinnacle to others. Greatness in turn becomes the people with such a tremendous sensibility that they can incorporate many, many wonderful things and, in the process, underline some deeper truth.
We're talking about both (1) his presentation and (2) his programming skills. Other kids should be encouraged by his presentation, instead of thinking, "Cool, but unfortunately he's a genius (or super-special, or etc.); I could never do that myself."